Expert witness: Ten times, and in controlled circumstances, a single drop of the defendant’s blood was allowed to fall onto the fabric. And in all ten cases, the stained area was much less than the expected 9.5 cm2. In fact, the stained area was always between 4.5 and 4.8 cm2. I conclude that a single drop of the defendant’s blood stains much less than 9.5 cm2 of the fabric.
Summarize Argument: Phenomenon-Hypothesis
The expert witness presents the hypothesis that a single drop of the defendant’s blood stains much less than 9.5 cm2 of the fabric. This hypothesis is supported by an observation that, in each of ten controlled tests, a drop of the defendant’s blood stained a much smaller area—only 4.5 to 4.8 cm2.
Notable Assumptions
The expert witness assumes that the controlled conditions of the ten tests are relevantly similar to (presumably) the crime scene. In other words, the expert assumes that the defendant’s blood would not have stained a much larger area in different, more relevant conditions.
The expert also assumes that ten tests are a large enough sample size to know how much fabric will be stained by a drop of blood. In other words, the expert assumes that more tests would not have changed the results.
A
If similar results had been found after 100 test drops of the defendant’s blood, the evidence would be even stronger.
This does not weaken the expert’s argument. Just because the evidence would have been stronger with 100 tests, that doesn’t mean that ten tests were too few. This doesn’t undermine the adequacy of the expert’s observations.
B
Expert witnesses have sometimes been known to fudge their data to accord with the prosecution’s case.
This does not weaken the expert’s argument. Some experts sometimes fudging their data tells us nothing about this particular expert, nor the quality of the expert’s observations. Like (E), this is just a weak attempt at an ad hominem attack.
C
In an eleventh test drop of the defendant’s blood, the area stained was also less than 9.5 cm2—this time staining 9.3 cm2.
This weakens the expert’s argument because it suggests that the ten tests may not be a reliable sample of how much fabric the defendant’s blood stains. In other words, this undermines the expert’s assumption that ten tests are enough.
D
Another person’s blood was substituted, and in otherwise identical circumstances, stained between 9.8 and 10.6 cm2 of the fabric.
This does not weaken the expert’s argument, because the argument is only concerned with defendant’s blood behaves, not anyone else’s blood. If anything, this shows that the test is able to show if someone’s blood stains a larger area, and is therefore more reliable.
E
Not all expert witnesses are the authorities in their fields that they claim to be.
This does not weaken the expert’s argument. Like (B), this is an attempt at an ad hominem attack on the expert. However, just some experts not truly being authorities tells us nothing about this particular expert or this particular argument.