Two Stopwatches, One Grade and a 0.6 Second Gap
A district in Punjab wants to grade 240 Class XI students on speed for the practical record file. Two physical education teachers time a 50 m sprint with handheld stopwatches. On the same morning the same student, Jaskaran, is timed at 6.8 seconds by one teacher and 7.4 seconds by the other. The school owns electronic timing gates, but they are used only at the district final. Each student is tested once, on a grass field, in the last period of the day, after a two-hour games session. The cut-off for an A grade is set at 7.0 seconds. The first timing gives Jaskaran an A; the second does not. His parents insist he trained hard all year, and the principal suggests using the better timing to avoid complaints.
1. Identify the sources of error in this testing and explain why these data cannot support a grade cut-off.
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Sources of error:
- Tester error: two different people, each starting and stopping a watch by hand, with no standardised starting signal. Hand-timed sprints carry a typical error of 0.1 to 0.3 seconds, and inter-tester disagreement is the biggest single cause. A 0.6 second gap between two timers on the same run is far larger than normal error, so at least one of the readings is not a valid measurement.
- Subject error: testing in the last period after a two-hour session guarantees fatigue; no standardised warm-up means arousal and muscle temperature vary between students.
- Protocol and environment: grass surface, unknown wind, no fixed lane markings, footwear not controlled, only one trial, no fixed rest interval.
- Instrumentation: no electronic gates, so the outcome depends on human reaction time on both the start and the stop.
Why the cut-off cannot stand: a decision threshold at 7.0 seconds is meaningless when the measuring instrument is uncertain by roughly 0.3 seconds. Jaskaran's true time could plausibly lie anywhere between about 6.7 and 7.5 seconds. The result is unreliable and also not comparable between students, so it cannot be used to award a grade. Using the better timing because the student trained hard is criterion contamination: effort is not a measurement, and choosing the favourable value destroys whatever little objectivity the test had.
2. Design a defensible re-test protocol and explain how the grade should be decided.
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Re-test protocol:
- Use the electronic gates if they can be borrowed; if not, one designated tester for all sprint timings, with a second tester as a cross-check and the mean of the two used.
- Flat, measured, marked 50 m on the same surface for every student; note wind and record it.
- Standardised warm-up of the same duration and content, and test at the same time of day, not after a long games session.
- Three trials with three to five minutes of recovery; record the best trial, plus the mean for the record file.
- Same footwear and starting command for everyone, with a flying or standing start specified in advance.
- Record raw data, tester names, date and conditions in the record file so that the result is auditable.
Deciding the grade: apply the agreed cut-off to the re-test. If the re-test falls within measurement error of 7.0 seconds, do not decide on speed alone: either place the student in the higher band with a note, or combine speed with a second valid indicator such as a standing broad jump or a 30 m fly time, weighted in advance. The principal should be told that the original data are unusable and why, and the decision recorded. Effort and parental pressure must play no part in the measurement decision.
3. Explain reliability, validity and objectivity using this case, and state which of the three failed and how.
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Reliability is consistency and repeatability: the same student tested again under the same conditions should produce a similar score. Here it failed at the first hurdle - one trial, no standardisation, and two timings of the same run 0.6 seconds apart. A test that cannot repeat itself cannot grade anyone.
Objectivity is agreement between different testers or scorers. It failed directly: two trained teachers watching the same sprint produced scores that straddle the grade boundary. Objectivity is usually strengthened by clear written protocols, mechanical or electronic measurement, and independent scorers.
Validity is whether the test measures what it claims. A 50 m sprint is a defensible measure of speed, so the test was chosen reasonably; but validity is undermined when the conditions differ from student to student, when students are tested while exhausted, and when the score is used to decide a grade near a boundary the instrument cannot resolve. A test can be valid in principle and still produce invalid results in practice.
The teacher's conclusion should be that the sprint test is acceptable, the administration was not, and the fix is procedural rather than a new test.