Scientific Reasoning in Crime Scene Investigation

• Separate an observation from a hypothesis and a prediction.
• Choose examinations that can distinguish relevant alternatives.
• Explain why a consistent finding does not prove one explanation.

An observation describes what was recorded and how: a fragment was recovered from a particular location, or a measurement was obtained using a stated procedure. A hypothesis proposes an explanation. A prediction states what observations would be expected if that explanation were true under specified conditions.

Use precise descriptions before causal language. “A red-brown stain” and “blood from an assault” are different statements. Appropriate tests may address the material’s identity, but identifying it does not independently explain the activity that deposited it.

A useful observation record identifies the item, its position, the recording method and the conditions under which it was examined. Distinguish a direct observation from a person’s account, an instrument output and a value calculated from other measurements. Each can be informative, but their sources and uncertainties differ.

Photographs and diagrams support the notes rather than replace their meaning. A photograph may show a stain’s appearance without establishing its chemical identity. A measurement should identify its reference points and units. If an item was moved before its original position was recorded, state that limitation instead of assigning the later position to the original scene.

Keep descriptions precise enough to revisit. “Possible impact mark at the lower edge” preserves uncertainty better than an unqualified statement that a named object caused it. Later examination may justify a more specific description. Preserve the earlier record and the reason for the change.

A useful alternative explains the relevant observations without merely changing a name. If the question concerns whether damage occurred before or after an item was moved, define the proposed sequence, surfaces and time window. Those details determine which observations might distinguish the accounts.

Alternative explanations are not automatically equally probable, equally well supported or exhaustive. Their purpose is to expose what the findings can and cannot distinguish. Do not invent an implausible alternative simply to make the preferred explanation look stronger.

Write the issue in one sentence before selecting tests. If the dispute is whether two fragments share an origin, concentrate on material properties and variation. If the dispute is whether contact happened during a particular event, the material comparison alone will not answer it. Timing, transfer and later handling become relevant.

A prediction is most useful when the possible observations differ between relevant explanations. Suppose both explanations predict a common material at a location. Confirming that material may be analytically useful but does little to distinguish the explanations. A different distribution or characteristic may be more informative, provided its expected behaviour is scientifically supported.

Plan how to interpret more than one possible result. State in advance what would weaken the preferred explanation, which result would be inconclusive and which assumptions require checking. This makes it harder to redesign the question after seeing the outcome simply to preserve the original narrative.

Ask what result would change the assessment. An examination that produces the same expected result under both alternatives may confirm a material property without helping distinguish the disputed event. More measurements are not necessarily more informative measurements.

Identify the appropriate specimens, reference material, controls and conditions before testing. A reconstruction can answer a useful question only within the conditions it represents. A demonstration using a different surface, load or environment may not justify a conclusion about the original event.

Record sampling decisions, unsuitable material and areas not examined. A result from one selected location does not automatically represent an entire room, object or population. Changes introduced by collection or testing should remain visible in the record.

A control checks a defined part of a procedure. A blank may help investigate unwanted material introduced during that procedure; a suitable positive control may demonstrate that a detection process can respond as intended. Neither establishes that every earlier collection step was correct or that every case sample behaves like the control.

Choose a method with demonstrated performance for the material and question. Calibration of an instrument concerns its measurement response; validation concerns whether the method is fit for its intended use. Operator competence, specimen condition and the interpretation procedure remain relevant. A familiar instrument is not automatically a suitable test for every question.

A reconstruction should specify what it represents and what it changes. Surface properties, object geometry, force, environmental conditions and intervening disturbance may matter. If a parameter is unknown, examine the effect of plausible variation rather than selecting one convenient value and treating the demonstration as a replay of the event.

If observations agree with a prediction, the hypothesis may remain plausible, but another explanation may predict the same observations. Agreement alone does not logically prove the original hypothesis. Conversely, a contradiction needs investigation: it may challenge the hypothesis, a supporting assumption, the measurement or the conditions represented.

An absent finding is informative only in relation to the chance that relevant material would have been present, survived and been recovered by the method used. “No fibres recovered” is not equivalent to “no contact occurred.” A negative result can be important, but its significance needs a justified expectation of detection.

Before interpreting non-detection, identify the chain of conditions needed for detection. Relevant material would need to be deposited, survive, be included in the recovered sample and produce a detectable response. A failure at any stage can affect what the examination observes.

The result may still discriminate between explanations if reliable information supports different expectations of detection. The point is to make that basis explicit. Do not assume either that absence always proves nothing or that a negative test automatically excludes an event.

• Was the relevant area actually examined?
• Could cleaning, weather or later use have changed it?
• Was the recovery method suitable for the material and surface?
• Did the test answer the proposed question at its demonstrated sensitivity?

When new information changes the question or an assumption, revisit the interpretation. Record what changed and why. Revising a hypothesis in response to evidence is appropriate; rewriting the earlier notes to make the answer appear obvious from the beginning is not.

A concise reasoning record identifies the question, observations, alternatives, examinations, results and limitations. Another examiner should be able to locate the basis of the conclusion and the points where a different assumption would change it.

Evidence Interpretation: Source and Activity Questions. Crime Scene Reconstruction.

Read the conclusion without the case narrative, then trace each claim back to an observation, a method or an explicit assumption. A claim about a material’s identity should have an analytical basis. A claim about an event needs an explanation of how the findings support it over relevant alternatives.

Check for hidden changes in wording. “Consistent with” can quietly become “came from,” and “could have occurred” can become “did occur.” Those are stronger claims. Their strength must come from additional evidence and justified reasoning, not from shortening a report.

An unresolved result is a legitimate outcome. State what information could help and whether it can realistically be obtained. Do not recommend endless examinations simply to avoid saying that the surviving evidence cannot answer the question.

• State observations before causal conclusions.
• Use relevant alternatives and examinations that can distinguish them.
• Assess negative findings against justified detection expectations.
• Record assumptions, limits and changes in interpretation.