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NS RWM8 study materials
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NS RWM8 practice questions

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Question 1Unverified content domain 1 (pending official confirmation)

In an experiment testing whether sleep duration affects reaction time, what is the independent variable?

  1. The time of day the test is given
  2. Reaction time
  3. The number of participants
  4. Sleep duration Correct answer.

Explanation

The independent variable is the factor the researcher manipulates or groups by, which here is sleep duration. Reaction time is the outcome that is measured, so it is the dependent variable. Sample size and testing schedule are design choices rather than the manipulated factor.

Question 2Unverified content domain 1 (pending official confirmation)

A scale consistently reports the same weight each time a person steps on it, but it reads 3 kg too high. Which statement best describes the scale?

  1. It is valid but not reliable
  2. It is neither reliable nor valid
  3. It is both valid and reliable
  4. It is reliable but not valid Correct answer.

Explanation

Reliability refers to consistency across repeated measurements, while validity refers to accuracy in measuring the intended construct. A scale that always gives the same reading but is systematically offset is consistent yet inaccurate. Therefore it is reliable but not valid.

Question 3Unverified content domain 1 (pending official confirmation)

Which variable is measured on an ordinal scale?

  1. Number of clinic visits in a year
  2. Blood type (A, B, AB, O)
  3. Body temperature measured in degrees Celsius
  4. Pain rated as mild, moderate, or severe Correct answer.

Explanation

Ordinal scales rank categories in a meaningful order without assuming equal distances between ranks. Pain categories such as mild, moderate, and severe are ordered but the gaps are not equal units. Temperature is interval, counts are ratio, and blood type is nominal.

Question 4Unverified content domain 1 (pending official confirmation)

A researcher divides a hospital's nursing staff into day, evening, and night shifts, then randomly selects nurses from each shift group in proportion to its size. Which sampling method is used?

  1. Snowball sampling
  2. Convenience sampling
  3. Simple random sampling without strata
  4. Stratified random sampling Correct answer.

Explanation

Stratified random sampling divides the population into strata, such as shift groups, and then draws random samples within each stratum. Selecting in proportion to stratum size preserves the population composition. Simple random sampling would not use predefined strata.

Question 5Unverified content domain 1 (pending official confirmation)

A study finds that coffee drinkers have higher rates of heart disease. Coffee drinkers in the sample are also more likely to smoke. What is smoking in this example?

  1. A moderating variable
  2. Confounding variable Correct answer.
  3. The dependent variable
  4. A mediating variable

Explanation

A confounder is associated with both the exposure and the outcome and can distort the observed relationship. Smoking is linked to coffee drinking and independently raises heart disease risk, so it may explain part of the association. Dependent, mediating, and moderating variables play different roles in a causal model.

Question 6Unverified content domain 1 (pending official confirmation)

Before enrolling participants in a study, a researcher explains procedures, risks, and the right to withdraw, then obtains written agreement. Which ethical principle is being applied?

  1. Debriefing
  2. Anonymity
  3. Confidentiality of records
  4. Informed consent Correct answer.

Explanation

Informed consent requires researchers to disclose procedures, risks, and the voluntary right to withdraw before participation. Obtaining written agreement after this disclosure is the defining step. Confidentiality and anonymity concern how data and identity are protected, not enrollment.

Question 7Unverified content domain 1 (pending official confirmation)

A study reports p = 0.03 with alpha set at 0.05. What is the most appropriate interpretation?

  1. The treatment effect size is large
  2. The result is not statistically significant at the 5% level
  3. There is a 3% probability that the null hypothesis is true
  4. The null hypothesis is rejected at the 5% level Correct answer.

Explanation

A p value below the predetermined alpha leads to rejecting the null hypothesis at that significance level. The p value is the probability of obtaining data at least as extreme if the null were true, not the probability that the null is true. Statistical significance also says nothing by itself about effect size.

Question 8Unverified content domain 1 (pending official confirmation)

When appraising a systematic review, which feature most directly reduces the risk of selective reporting of studies?

  1. Registering the review protocol in advance Correct answer.
  2. Restricting inclusion to studies with positive findings
  3. Including only studies published in English
  4. Relying on a single database search

Explanation

Registering the protocol in advance fixes the review question, eligibility criteria, and outcomes before results are known. This transparency reduces the chance that reviewers will selectively report or emphasize favorable findings. Language limits, positive-result filters, and single-database searches tend to increase bias.

Question 9Unverified content domain 1 (pending official confirmation)

A randomized trial with high internal validity uses a narrow sample of healthy volunteers aged 20–30. Which threat is most relevant?

  1. Instrumentation drift
  2. Demand characteristics
  3. Selection bias within randomization
  4. Limited external validity Correct answer.

Explanation

A narrow, healthy volunteer sample can produce strong internal validity through randomization while limiting applicability to other patients. That limitation is a threat to external validity, or generalizability. Randomization addresses selection bias, and instrumentation or demand characteristics concern measurement and procedure.

Question 10Unverified content domain 1 (pending official confirmation)

Which design provides the strongest evidence for a causal claim about a new treatment?

  1. A case series
  2. A cross-sectional survey
  3. A randomized controlled trial Correct answer.
  4. A retrospective cohort study

Explanation

Randomization balances known and unknown confounders between groups, which supports causal inference. Cross-sectional surveys cannot establish temporal order, case series lack a comparison group, and retrospective cohorts remain vulnerable to confounding. Therefore the randomized controlled trial gives the strongest causal evidence.

Question 11Unverified content domain 1 (pending official confirmation)

Which statement best describes the null hypothesis in a quantitative study?

  1. It summarizes the observed effect size obtained from the study sample.
  2. It states that no difference or association exists in the population. Correct answer.
  3. It predicts the exact value of the sample mean that the researcher expects.
  4. It is accepted whenever the calculated p value is greater than alpha.

Explanation

The null hypothesis specifies that there is no difference or association in the population and provides the baseline that statistical tests evaluate. It is not a prediction about the sample mean, and failing to reject it does not prove that it is true. Rejection supports the alternative hypothesis but does not establish it with certainty.

Question 12Unverified content domain 1 (pending official confirmation)

At the start of 2024, a town has 10,000 adults, of whom 500 already have diabetes. During 2024, 50 new cases are diagnosed among the 9,500 adults who did not have diabetes at baseline. What is the incidence rate for 2024?

  1. 500 divided by 10,000
  2. 550 divided by 10,000
  3. 50 divided by 9,500 Correct answer.
  4. 50 divided by 10,000

Explanation

Incidence uses new cases during a period divided by the population at risk at the start of that period. The 500 existing cases are not at risk for becoming incident cases, so the denominator is the 9,500 adults without diabetes at baseline. This yields about 5.3 new cases per 1,000 adults for the year.

Question 13Unverified content domain 1 (pending official confirmation)

What is the primary purpose of blinding participants and outcome assessors in a clinical trial?

  1. To remove the need for random assignment to the treatment groups.
  2. To reduce bias arising from expectations about the treatment received. Correct answer.
  3. To ensure that every participant receives the treatment that is most effective.
  4. To increase the statistical power of the trial by enlarging the effective sample.

Explanation

Blinding conceals treatment allocation so that participants, clinicians, and assessors cannot let expectations influence reporting, care, or measurement. It protects the trial from performance and detection bias but does not increase sample size or replace randomization. Random assignment and blinding serve complementary purposes.

Question 14Unverified content domain 1 (pending official confirmation)

A trial with a small sample fails to detect a true difference between two treatments. Which error has occurred?

  1. A Type II error, because a true effect was not detected. Correct answer.
  2. A standard error, because the estimate varied across samples.
  3. A sampling error, because the sample was not representative.
  4. A Type I error, because a real difference was missed.

Explanation

A Type II error occurs when a statistical test fails to reject a false null hypothesis, which is what happens when a real treatment difference is missed. Small samples and low power increase the risk of this error. A Type I error is the opposite mistake: rejecting a null hypothesis that is actually true.

Question 15Unverified content domain 1 (pending official confirmation)

A 95% confidence interval for a mean difference between two groups is (0.2, 4.8). Which interpretation is most appropriate?

  1. The interval excludes zero, so the result is significant at the 0.05 level. Correct answer.
  2. Exactly 95% of study participants had differences between 0.2 and 4.8.
  3. There is a 95% probability that the true mean difference lies inside this specific interval.
  4. The study would produce a significant result in exactly 95% of future replications.

Explanation

Because the entire interval lies above zero, zero is not a plausible value for the mean difference at the 95% confidence level, which corresponds to statistical significance at a two-sided alpha of 0.05. The 95% refers to the long-run coverage of the procedure across repeated samples, not the probability that this fixed interval contains the true parameter. Individual participant differences are not described by a confidence interval for a mean.

Question 16Unverified content domain 1 (pending official confirmation)

Which measure of association is most appropriate for a case-control study of a rare disease?

  1. Risk ratio, because it directly estimates the risk of disease in each exposure group.
  2. Prevalence ratio, because it compares existing cases across exposure groups.
  3. Incidence rate, because it accounts for person-time at risk during follow-up.
  4. Odds ratio, because participants are selected on the basis of disease status. Correct answer.

Explanation

In a case-control study, participants are sampled by outcome status, so the incidence of disease in the source population cannot be estimated directly and a risk ratio is generally unavailable. The odds ratio compares the odds of exposure among cases with the odds among controls and approximates the risk ratio when the disease is rare. Person-time incidence rates require a cohort design with follow-up.

Question 17Unverified content domain 1 (pending official confirmation)

In a randomized controlled trial, some participants stop taking their assigned treatment. Why is intention-to-treat analysis generally preferred?

  1. It removes the need for a control group in the trial.
  2. It preserves the benefits of randomization and avoids bias from selective dropout. Correct answer.
  3. It guarantees that every participant receives the correct dose of the intervention.
  4. It increases the apparent treatment effect by excluding participants who did not adhere.

Explanation

Intention-to-treat analysis compares participants according to the group to which they were originally randomized, regardless of adherence. This preserves the comparability created by randomization and prevents bias that can arise when participants who drop out or stop treatment differ between groups. Excluding nonadherent participants can distort the estimated effect.

Question 18Unverified content domain 1 (pending official confirmation)

A qualitative researcher continues interviewing participants until no new themes or insights emerge from the data. What is this called?

  1. Bracketing, because the researcher sets aside personal preconceptions.
  2. Data saturation, because further collection yields no new themes. Correct answer.
  3. Triangulation, because multiple data sources are combined to confirm findings.
  4. Member checking, because participants review the researcher's interpretations.

Explanation

Data saturation is the point at which additional interviews or observations no longer produce new themes or meaningful insights. It is used to judge when sampling in qualitative research is sufficient. Triangulation, member checking, and bracketing are different strategies for enhancing rigor or reflexivity.

Question 19Unverified content domain 1 (pending official confirmation)

A trial has 80% power at a two-sided alpha of 0.05 to detect a specified clinically meaningful difference. Which statement is most accurate?

  1. There is an 80% probability that the null hypothesis is actually true.
  2. The trial has an 80% chance of significance regardless of the true effect.
  3. The probability of making a Type I error in the trial is 20%.
  4. Given that true effect, 80% of similar trials would reject the null hypothesis. Correct answer.

Explanation

Statistical power is the probability of rejecting the null hypothesis when the true effect equals the specified alternative value. It is not the probability that the null hypothesis is true, and it does not apply regardless of the true effect. The Type I error rate is set by alpha, which is 0.05 here, not 20%.

Question 20Unverified content domain 1 (pending official confirmation)

A meta-analysis reports an I² value of 75%. What does this most likely indicate?

  1. Seventy-five percent of the included studies found a statistically significant effect.
  2. There is substantial inconsistency among study effects beyond chance. Correct answer.
  3. The pooled effect is 75% likely to be clinically important in practice.
  4. Publication bias is present in 75% of the included studies.

Explanation

I² estimates the proportion of variability in effect estimates that is due to heterogeneity rather than sampling error. A value of 75% is commonly interpreted as substantial inconsistency, so the pooled estimate should be interpreted cautiously and sources of heterogeneity explored. I² does not measure publication bias or the probability that an effect is clinically important.

Question 21Unverified content domain 1 (pending official confirmation)

A study of hospitalized older adults is designed to maximize possible benefits and minimize possible harms. Which ethical principle does this primarily reflect?

  1. Veracity
  2. Respect for autonomy
  3. Justice
  4. Beneficence Correct answer.

Explanation

Beneficence requires researchers to act for participants' welfare by weighing potential benefits against risks. Justice concerns the fair distribution of research burdens and benefits, while autonomy concerns self-determination and veracity concerns truthfulness.

Question 22Unverified content domain 1 (pending official confirmation)

Which study design identifies a group by exposure status and then follows them forward to observe whether an outcome occurs?

  1. Cross-sectional study
  2. Ecological study
  3. Prospective cohort study Correct answer.
  4. Case-control study

Explanation

A prospective cohort study begins with exposure status and follows participants over time to observe incident outcomes. A case-control study starts with outcome status, a cross-sectional study measures exposure and outcome at the same time, and an ecological study uses group-level data.

Question 23Unverified content domain 1 (pending official confirmation)

A researcher obtains an alphabetical list of clinic patients and selects every eighth patient for interviews. Which sampling method is being used?

  1. Cluster sampling
  2. Simple random sampling
  3. Systematic sampling Correct answer.
  4. Convenience sampling

Explanation

Systematic sampling selects participants at fixed intervals from an ordered list, usually after choosing a random starting point. Simple random sampling gives each person an independent chance of selection, convenience sampling uses readily available people, and cluster sampling selects whole groups.

Question 24Unverified content domain 1 (pending official confirmation)

Which variable is measured on a ratio scale?

  1. Waiting time in minutes Correct answer.
  2. Blood type classified as A, B, AB, or O
  3. Pain severity rated as mild, moderate, or severe
  4. Oral temperature measured in degrees Celsius

Explanation

A ratio scale has equal intervals and a true zero, so waiting time in minutes permits meaningful ratios such as one wait being twice as long as another. Blood type is nominal, pain categories are ordinal, and Celsius temperature has equal intervals but an arbitrary zero point.

Question 25Unverified content domain 1 (pending official confirmation)

In a cohort study of occupational exposure and respiratory disease, participants who developed disease recall their workplace exposures more thoroughly than those who remained healthy. Which bias is most likely?

  1. Recall bias Correct answer.
  2. Sampling bias
  3. Attrition bias
  4. Observer bias

Explanation

Recall bias occurs when comparison groups differ in how accurately or completely they remember past exposures, often because illness prompts closer scrutiny of prior events. Attrition bias involves differential dropout, observer bias involves biased outcome assessment, and sampling bias concerns who enters the sample.

Question 26Unverified content domain 1 (pending official confirmation)

A randomized trial finds that a fall-prevention program reduces falls in older women but has little effect in older men. What term best describes sex in this situation?

  1. Mediator
  2. Selection variable
  3. Confounder
  4. Effect modifier Correct answer.

Explanation

An effect modifier is a variable across which the magnitude or direction of an intervention's effect differs; here the program's effect varies by sex. A confounder is associated with exposure and outcome and distorts the association, while a mediator lies on the causal pathway between exposure and outcome.

Question 27Unverified content domain 1 (pending official confirmation)

A survey is mailed to 1,000 randomly selected adults, but adults with poorer health are less likely to return it. What is the most likely consequence?

  1. Nonresponse bias may distort the estimates Correct answer.
  2. Random measurement error decreases
  3. The margin of error is eliminated
  4. The sample becomes more representative

Explanation

Nonresponse bias arises when people who respond differ systematically from those who do not, especially on the study outcome. Random selection does not remove bias caused by differential participation, and it does not eliminate sampling error.

Question 28Unverified content domain 1 (pending official confirmation)

A screening test has a sensitivity of 95%. What does this value most directly mean?

  1. 95% of all positive results are true positives
  2. 95% of people with the disease test positive Correct answer.
  3. 95% of people without the disease test negative
  4. 95% of the population will be correctly classified

Explanation

Sensitivity is the proportion of truly diseased people who test positive, so a high value means there are few false negatives. The proportion of disease-free people testing negative is specificity, and the proportion of positive results that are true positives is the positive predictive value.

Question 29Unverified content domain 1 (pending official confirmation)

A single-group study measures depression before and after a 12-week exercise program and finds improvement. Participants also recover gradually from a seasonal stressful period during the same 12 weeks. Which threat to internal validity is most directly illustrated?

  1. Testing effect
  2. Instrumentation
  3. Regression to the mean
  4. Maturation Correct answer.

Explanation

Maturation refers to naturally occurring changes over time, such as gradual recovery, that can be mistaken for an intervention effect in a single-group design. Instrumentation concerns changes in measurement, testing concerns repeated assessment, and regression to the mean concerns extreme scores moving toward average.

Question 30Unverified content domain 1 (pending official confirmation)

A large trial finds a 1-point improvement on a 100-point quality-of-life scale, with p < 0.001. Which conclusion is most appropriate?

  1. The result proves the treatment is ineffective
  2. The result cannot be interpreted without a confidence interval for the sample mean
  3. The result is statistically significant but may not be clinically meaningful Correct answer.
  4. The result is clinically important because p is very small

Explanation

A very small p value indicates that the observed difference is unlikely under the null hypothesis, not that the difference is large or important. Clinical meaningfulness depends on the size of the effect relative to a prespecified threshold, and a 1-point change may be trivial.

Question 31Unverified content domain 1 (pending official confirmation)

In a randomized controlled trial, what is the main purpose of randomly allocating participants to treatment groups?

  1. To ensure that participants remain unaware of their assigned treatment
  2. To increase statistical power by enlarging the effective sample size
  3. To guarantee that the sample is representative of the general population
  4. To make the groups comparable on known and unknown prognostic factors Correct answer.

Explanation

Randomization distributes measured and unmeasured prognostic factors between groups on average, so outcome differences can be attributed more confidently to the treatment. It does not ensure blinding, representativeness, or a larger effective sample.

Question 32Unverified content domain 1 (pending official confirmation)

A researcher surveys a random sample of adults at a single clinic visit to estimate the current prevalence of hypertension. Which study design is this?

  1. Prospective cohort study
  2. Randomized controlled trial
  3. Cross-sectional study Correct answer.
  4. Case-control study

Explanation

A cross-sectional study measures exposure and outcome at one point in time and is commonly used to estimate prevalence. Cohort studies follow participants forward, case-control studies sample by outcome status, and trials test interventions.

Question 33Unverified content domain 1 (pending official confirmation)

A trial reports a p-value of 0.04 for the difference between treatments. Which interpretation is most appropriate?

  1. There is a 4% probability that the null hypothesis is true in the underlying population.
  2. Data this extreme would occur 4% of the time if the null hypothesis were true. Correct answer.
  3. There is a 96% probability that a repeat trial will find a statistically significant result.
  4. The treatment difference is large enough to be clearly clinically important.

Explanation

A p-value is the probability of observing a result at least as extreme as the one obtained, assuming the null hypothesis is true. It is not the probability that the null is true, a replication probability, or a measure of clinical importance.

Question 34Unverified content domain 1 (pending official confirmation)

In a trial, the event rate is 10% in the control group and 5% in the treatment group. What is the number needed to treat to prevent one additional event?

  1. 10
  2. 5
  3. 15
  4. 20 Correct answer.

Explanation

The absolute risk reduction is 10% minus 5%, or 5 percentage points. The number needed to treat is 1 divided by 0.05, which equals 20.

Question 35Unverified content domain 1 (pending official confirmation)

A cohort study finds that coffee drinking is associated with lung cancer. Smokers are more likely to drink coffee and to develop lung cancer. What is smoking in this example?

  1. A confounder Correct answer.
  2. A mediator
  3. An effect modifier
  4. A collider

Explanation

A confounder is associated with the exposure and independently affects the outcome, distorting the observed exposure-outcome association. Smoking fits both conditions because it is linked to coffee drinking and causes lung cancer.

Question 36Unverified content domain 1 (pending official confirmation)

In a randomized trial, 30% of participants in the intervention arm and 5% in the control arm drop out before the outcome is measured. Which bias is most likely?

  1. Selection bias at randomization
  2. Attrition bias Correct answer.
  3. Recall bias
  4. Observer bias

Explanation

Differential loss to follow-up can leave the remaining groups unbalanced and is called attrition bias. Randomization protects against selection bias at baseline, while recall and observer bias concern how measurements are made.

Question 37Unverified content domain 1 (pending official confirmation)

A qualitative researcher deliberately selects adults who have recently experienced a traumatic amputation because they can provide rich information about rehabilitation. Which sampling strategy is this?

  1. Snowball sampling
  2. Convenience sampling
  3. Purposive sampling Correct answer.
  4. Simple random sampling

Explanation

Purposive sampling selects participants for specific characteristics relevant to the research question and for rich information. Convenience sampling uses whoever is available, snowball sampling relies on referrals, and random sampling selects by chance.

Question 38Unverified content domain 1 (pending official confirmation)

Why can a case-control study not directly estimate the incidence of a disease?

  1. It excludes people who are already immune to the disease.
  2. Outcome status determines selection rather than population risk. Correct answer.
  3. Exposure status cannot be measured accurately in retrospect.
  4. It follows participants only after the outcome has already occurred.

Explanation

Case-control sampling is based on outcome status, so the ratio of cases to controls is fixed by the investigator rather than by disease occurrence in the population. Incidence requires following a population at risk over time.

Question 39Unverified content domain 1 (pending official confirmation)

A screening test has 90% sensitivity and 90% specificity. In a population with a disease prevalence of 1%, what is the approximate positive predictive value?

  1. About 90%
  2. About 99%
  3. About 50%
  4. About 8% Correct answer.

Explanation

In 1,000 people, about 10 have disease: 9 are true positives and 1 is a false negative. Among the 990 without disease, 99 are false positives, so the positive predictive value is about 9 divided by 108, or 8%.

Question 40Unverified content domain 1 (pending official confirmation)

A nonrandomized study compares a new drug with usual care, but sicker patients are more likely to receive the new drug. Which method best addresses this confounding by indication?

  1. Using a narrower confidence interval
  2. Applying a random-number table to the outcome data
  3. Increasing the sample size
  4. Propensity score matching Correct answer.

Explanation

Propensity scores estimate each participant's probability of receiving treatment based on measured covariates, allowing matched or weighted groups with similar baseline risk. Increasing sample size or narrowing an interval does not remove confounding by indication.

Question 41Unverified content domain 1 (pending official confirmation)

A pain scale produces almost identical scores when the same patient is assessed twice on the same day, but on average it overstates pain by 3 points compared with a structured clinical interview. Which statement best describes the scale?

  1. It is reliable but not valid. Correct answer.
  2. It is neither reliable nor valid.
  3. It is valid but not reliable.
  4. It is both reliable and valid.

Explanation

Reliability concerns consistency across repeated measurements, whereas validity concerns whether an instrument accurately measures the intended construct. A scale that yields stable but systematically inflated scores is consistent yet biased, so it lacks validity.

Question 42Unverified content domain 1 (pending official confirmation)

A cross-national analysis reports that countries with higher average sugar consumption also have higher diabetes prevalence. A news article concludes that individuals who eat more sugar are more likely to develop diabetes. Which error does this conclusion illustrate?

  1. Ecological fallacy Correct answer.
  2. Regression to the mean
  3. Selection bias
  4. Recall bias

Explanation

An ecological study compares aggregate group-level data and cannot establish that the exposure–outcome association holds for individuals within those groups. Inferring individual-level risk from group averages is called the ecological fallacy.

Question 43Unverified content domain 1 (pending official confirmation)

In a placebo-controlled trial of an injectable arthritis treatment, both participants and outcome assessors are unaware of treatment assignment. What is the primary reason for this design?

  1. To reduce bias from participant and assessor expectations Correct answer.
  2. To eliminate the need for informed consent
  3. To increase the statistical power of the trial
  4. To ensure the groups are prognostically similar at baseline

Explanation

Blinding prevents knowledge of assignment from influencing symptom reporting, use of co-interventions, and outcome assessment. It protects against expectation and detection bias, but it does not create baseline prognostic balance.

Question 44Unverified content domain 1 (pending official confirmation)

In a trial of a smoking-cessation program, some participants assigned to the program never attend sessions, and some controls attend outside programs. The investigators analyze all participants in their originally assigned groups. Which analysis strategy is described?

  1. Per-protocol analysis
  2. As-treated analysis
  3. Intention-to-treat analysis Correct answer.
  4. Propensity-score matched analysis

Explanation

Intention-to-treat analysis keeps participants in the groups to which they were randomized, regardless of adherence or crossover. This preserves the prognostic balance created by randomization and avoids bias from post-randomization selection.

Question 45Unverified content domain 1 (pending official confirmation)

A trial comparing two antihypertensives reports a mean systolic difference of 2.0 mmHg (95% CI, 1.1 to 2.9). Which interpretation is most appropriate?

  1. The true difference lies plausibly between 1.1 and 2.9 mmHg Correct answer.
  2. There is a 95% probability that the sample mean difference was exactly 2.0 mmHg
  3. Ninety-five percent of participants improved by between 1.1 and 2.9 mmHg during the trial
  4. The interval indicates that the analysis lacked adequate statistical power to detect a difference

Explanation

A 95% confidence interval is a range of plausible values for the population effect estimated from the sample. Because the entire interval lies above zero, the observed difference is statistically significant at the 0.05 level.

Question 46Unverified content domain 1 (pending official confirmation)

A screening program detects lung cancer earlier, and patients survive longer from the date of diagnosis, but the age at death is unchanged compared with unscreened patients. Which bias best explains the apparent survival benefit?

  1. Healthy-screenee bias
  2. Length bias
  3. Information bias
  4. Lead-time bias Correct answer.

Explanation

Lead-time bias occurs when earlier detection advances the date of diagnosis without postponing death, making measured survival appear longer. Comparing survival from diagnosis rather than from a common starting point exaggerates the benefit of screening.

Question 47Unverified content domain 1 (pending official confirmation)

In a case-control study of occupational solvent exposure and neuropathy, controls are individually matched to cases on age and duration of employment. What is the main purpose of this matching?

  1. To permit direct calculation of incidence rates
  2. To reduce misclassification of disease status
  3. To control confounding by the matched variables Correct answer.
  4. To raise the prevalence of exposure among controls

Explanation

Matching selects controls with the same values of the matching factors as their cases, removing the influence of those factors on the exposure–disease comparison. It addresses confounding by the matched variables rather than disease misclassification or incidence estimation.

Question 48Unverified content domain 1 (pending official confirmation)

A randomized trial with 40 participants per arm finds no statistically significant difference in fracture risk, although a clinically important difference may exist. Which explanation is most likely?

  1. The p-value was too small to detect a real difference
  2. Low statistical power makes a type II error possible Correct answer.
  3. The absence of significance proves the treatments are equivalent
  4. A type I error has occurred because the result was not significant

Explanation

Small samples provide limited power to detect real effects, so a non-significant result may reflect a type II error rather than true equivalence. Failing to reject the null hypothesis does not establish that no difference exists.

Question 49Unverified content domain 1 (pending official confirmation)

In a cohort study, exposure is measured with a questionnaire that misclassifies a similar proportion of people in both the diseased and non-diseased groups. Assuming the error is unrelated to the outcome, what is the most likely effect on the estimated relative risk?

  1. It is biased toward the null value Correct answer.
  2. It is biased away from the null value
  3. It is unaffected because the error is equal in both groups
  4. It reverses direction only if the outcome is rare

Explanation

Non-differential misclassification of a binary exposure tends to dilute the observed association, moving the relative risk toward 1.0. Equal error proportions in the outcome groups do not cancel out; they reduce the contrast between exposed and unexposed participants.

Question 50Unverified content domain 1 (pending official confirmation)

A study of triggers of myocardial infarction compares each patient's exposure during the hour before symptom onset with that same patient's exposure at other randomly selected times. Which design is this?

  1. Cross-sectional study
  2. Case-crossover study Correct answer.
  3. Nested case-control study
  4. Prospective cohort study

Explanation

A case-crossover design uses each participant as his or her own control, comparing exposure during a brief hazard window with exposure during control periods. This controls stable individual confounders such as age, sex, and chronic disease.