IAI CP2 common mistakes
IAI CP2 · Understand the traps and how to avoid them.
Treating the bloodstain pattern analysis questions as pure pattern-recognition and selecting an answer from the visual shape alone, without tying the pattern to the mechanism that produced it (impact, projection, transfer, flow, or respiration/expiration).
Candidates study stain photographs and memorise labels such as 'high-velocity impact spatter' or 'cast-off' as if the exam were a naming quiz. When a question describes a pattern in text or asks which mechanism best explains a mixed scene, the label-matching strategy breaks down because the discriminating information is the physical mechanism, not the outline.
What to do instead
For every pattern category, write a one-line mechanism statement (energy source, direction of force, typical droplet size range, distribution geometry) and practise classifying described scenes by mechanism first, label second. When a question offers two plausible labels, choose the one whose mechanism is consistent with the stated droplet size, angle, and distribution.
Assigning directional travel to stains by pointing the tail toward the direction of travel, or by using the long axis of an elliptical stain without checking which end is the parent drop versus the satellite or spine.
The visual intuition that 'the pointy end leads' is common in everyday objects but is backwards for bloodstain directionality; the scalloped or spined edge and the tail point in the direction the drop was moving. Candidates who rely on intuition rather than the documented convention invert directionality on every question that asks about travel direction.
What to do instead
Memorise the rule explicitly: the satellite spatter, spines, and the narrow tail are on the side of the parent stain toward which the drop was travelling. Practise with described stains (for example, a stain with spines on its left edge) and state the travel direction out loud before answering, and cross-check against the angle of impact geometry.
Computing the angle of impact with the wrong trigonometric relationship, for example using width divided by length rather than the arcsine of the width-to-length ratio, or measuring the ellipse along the wrong axis.
Candidates remember that width and length are involved and that some trig function applies, but confuse sine with tangent or invert the ratio. Measuring length as the minor axis or including the tail in the length measurement compounds the error, and the resulting angle is often plausible enough that the mistake goes unnoticed.
What to do instead
Fix the formula in memory as angle of impact = arcsine (width / length), where length is the major axis of the parent stain excluding any tail and width is the minor axis measured at the widest point. Work two or three numeric examples by hand until the values are familiar, and sanity-check that a perfectly circular stain gives 90 degrees and a very elongated stain gives a small angle.
Performing area-of-origin analysis by drawing lines through the long axis of each stain from the stain centre rather than from the calculated point of convergence, and by projecting in the correct plane only after failing to establish a consistent reference plane.
Stringing or line-drawing is often taught as a practical demonstration, and candidates reproduce the mechanics without understanding that each line must be drawn through the stain's long axis at the calculated angle of impact, in a plane parallel to the target surface, and extended to a common convergence point before the vertical projection. Mixing stains from different impact events or different surfaces also corrupts the convergence.
What to do instead
State the sequence as a checklist: select stains from a single event, measure width and length, compute angle of impact, draw the direction line in the plane of the surface, find the point of convergence, then project the origin above or below that point using the impact angles. Exclude stains with angles below roughly 20 degrees or above roughly 80 degrees when they distort the convergence, and justify exclusions.
Confusing impact spatter with other mechanisms that mimic it, such as expirated blood, cast-off, or gunshot-related back spatter, and defaulting to 'high-velocity impact spatter' whenever the droplets are small.
Droplet size alone is a weak discriminator; small droplets can arise from expiration, from a gunshot, from cast-off, or from high-energy impact. Candidates latch onto a remembered size threshold and ignore contextual clues such as location (mouth/nose area), presence of air bubbles or mucus, linear distribution, or association with a weapon.
What to do instead
Build a differential table for each mechanism with columns for droplet size, distribution shape, location, associated features, and typical triggering event. When a question mentions bubbles, mucus, or staining around the nose and mouth, consider expirated blood before impact spatter; when stains form a linear trail, consider cast-off; when stains cluster on a surface facing a wound, consider back spatter.
Overstating what bloodstain pattern analysis can establish, for example asserting that a specific pattern proves a particular sequence of events, a specific number of blows, or the identity of the person who struck.
Candidates conflate pattern classification with reconstruction of intent and sequence. The discipline supports statements about the physical mechanism, direction, and relative position of blood sources, but it does not reliably establish sequence in complex scenes, the number of blows, or who was holding an object. Overclaiming is a classic error in both exam answers and real casework.
What to do instead
Learn the boundary statements: pattern analysis can support mechanism, direction of travel, area of origin, and relative positions of victim and blood source; it generally cannot establish timing between separate events, exact blow counts, or identity. When an answer option claims certainty about sequence or intent, treat it as suspect unless the question supplies independent corroborating evidence.
Ignoring the effect of the target surface and environmental conditions when interpreting a pattern, and comparing droplet sizes across scenes with different substrates as if they were directly comparable.
Textbook droplet size ranges are derived under controlled conditions on a defined surface. Rough or porous surfaces produce satellite spatter and irregular stains; absorbent surfaces wick and distort; a hard smooth surface preserves fine detail. Candidates who ignore substrate effects misclassify patterns and misjudge impact energy.
What to do instead
Before answering any interpretation question, identify the substrate and note how it modifies the pattern (satellites on rough surfaces, wicking on fabric, distortion on curved surfaces). When a question contrasts two scenes, ask whether the difference could be explained by substrate rather than by mechanism, and prefer the answer that accounts for the surface.
Misapplying the terminology of void patterns, shadowing, and transfer stains, for example calling an absence of blood a 'void stain' without linking it to an intervening object, or treating a contact transfer as impact spatter.
These terms describe relationships between objects and blood, not stain morphology alone. Candidates memorise the words without the relational logic, so they fail questions that require inferring that an object was present during staining (void) or that an object contacted a bloody surface (transfer).
What to do instead
Define each term by its relational logic: a void is an absence of staining where an object blocked deposition, implying the object was present at the time of deposition; a transfer is blood moved from a wet bloody surface to another surface by contact, implying contact after wetting. Practise by describing what must have been true physically for the observed absence or contact.
Failing to distinguish arterial spatter from cast-off or impact spatter when a question describes a repeated, pulsating, or arcing distribution, and missing the implication that the blood source was under pressure.
Arterial damage produces spatter that can resemble cast-off in droplet size and can form linear or arcing trails as the person or limb moves. Candidates focus on the linear arrangement and select cast-off, overlooking the pulsatile pattern and the large volume that suggest a pressurized source.
What to do instead
Look for the signature of a pressurized source: repeated groupings consistent with a heartbeat, larger volumes, and distribution that follows body movement rather than weapon swing. Compare with cast-off, which originates from blood on a moving object and typically produces a trail aligned with the swing arc. State which feature distinguishes the two before selecting an answer.
Skipping documentation and chain-of-custody reasoning in questions that ask how a bloodstain scene should be recorded, photographed, or sampled, and answering only the analytical portion of a combined question.
Candidates treat the exam as pure physics and overlook that many questions test scene documentation standards: overall, mid-range, and close-up photography with and without scale, notes on substrate and lighting, and appropriate reference samples. Combined questions then fail on the procedural half even when the analysis is right.
What to do instead
Prepare a documentation checklist (overall, mid-range, close-up; scale in every evidentiary frame; orientation and substrate notes; control and reference samples; chain-of-custody entries) and rehearse applying it to described scenes. On combined questions, answer the procedural sub-question explicitly rather than assuming the analytical answer carries it.