Lecture #11 | Estimating PMI using Insect Succession Data

Successional Interval

Indicates the days following death that each species is found on the corpse

• Change in abundance of each taxon is shown by slope of the line

  

Occurence matrix

Presence/absence of a species over time

• a probability model with 1= present and 0=absent

Explain the occurrence matrix

Graph that explains the presence of necrophageous species A and B on 10 pig carcasses

• The use of ‘ indicates absence: A’B: just B

Based on the occurrence matrix, what is the PMI estimated to be if both A and B are present?

Days 2-3

explanation: If the specific species A and B are present on the body (A is determined to be an early colonizer), the body is probably fresh. So if 80-90% of cases have A and B, it is likely the body is fresh, so PMI is around day 2-3

Why is it not 100%: Insects are not 100% reliable, so 10-20% of cases are outside the norm

Based on the occurrence matrix, how many days since death is most likely if only species B is present?

Absent A, present B: A’B

Likely 5 days after death

Explanation: Days 4-6, mainly 5, represent the largest presence of just B, without A.

• 100% of carrion had just B on day 5

• 10% had just B on day 2

What constraints exist to using successional interval to estimate PMI?

1. Predictability of succession

   1. Not 100%, there will always be outliers

2. Reference data for comparison

   1. References are old cases but can be limited by locality in both geography and seasonality

How can forensic statistics be improved for better accuracy?

1. Combine multiple matrices for several species

2. Use both immature and adult presence