The basics
DNA analysis is based on the comparison of DNA undisputable and disputable profiles, and the result may be twofold:
- An undisputable DNA profile does not match or is not contained a disputable DNA profile. In this case that person is not a donor of the biological material, or
- The undisputable DNA profile matches or is contained in the disputed DNA profile. In this case, the person cannot be excluded as a donor of the biological material in question. The strength of the connection between the disputed and undisputed DNA profiles is obtained using biostatistical methods.
Hypotheses
Biostatistics is based on the comparison of two mutually exclusive hypotheses that cover the whole range of possibilities:
- Ht - Prosecutor's hypothesis (biological material in the biological trace is from the suspect);
- Ho - Defense hypothesis (donors of biological material are not related to the suspect).
The result of these comparisons is the Likelihood Ratio (LR), which is the main result of the DNA analysis.
Likelihood ratio
LR is the number of times it is more likely to get the results of forensic DNA analysis under Ht than under Ho.
The value of LR
Courts do not usually come across the concept of LR. For example, it is impossible to quantify the LR of witness statements (they can be more or less reliable or persuasive).
However, there is at least a need to make a verbal scale to describe how the LR value obtained by DNA expert evidence supports the claim of the prosecutor or defense attorney.
Verbalisation of LR values is a matter of convention. DNA center uses the scale used by the Forensic Science Service, which is published in the book "Forensic DNA evidence interpretation" by J. Buckleton, C.M. Triggs and S.J. Walsh.
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| 1.000.000 + | extremely strong | Sa izuzetno velikom sigurnošću se može tvrditi da je osoba donor biološkog materijala |
| 100.000 - 1.000.000 | very strong | Sa velikom sigurnošću se može tvrditi da je osoba donor biološkog materijala |
| 10.000 - 100.000 | strong | Sa sigurnošću se može tvrditi da je osoba donor biološkog materijala |
| 1.000 - 10.000 | moderately strong | Osoba se ne može isključiti kao donor biološkog materijala |
| 100 - 1.000 | moderate | Osoba se ne može isključiti kao donor biološkog materijala |
| 10 - 100 | weak | Osoba se ne može isključiti kao donor biološkog materijala |
| 1 - 10 | inconclusive | Osoba se ne može isključiti kao donor biološkog materijala |
