What does a likelihood ratio mean in forensic DNA evidence?

A likelihood ratio is the statistic forensic scientists use to express how strongly DNA findings support one explanation over another.

It compares the probability of observing a DNA result if one proposition is true - for example, that the accused contributed DNA to the sample - versus if an alternative proposition is true, such as another unknown individual contributed to the sample.

It does not measure guilt, determine how DNA was deposited, nor does the ratio consider family members unless specified.

For example, a high likelihood ratio such as 1 million may indicate strong support that a person contributed to a DNA profile at a source level. It does not establish when the DNA was deposited, how it came to be present, or whether it relates to the alleged activity.

Read an Australian case - When the “DNA 1 billion” statistic is wrong

Misinterpretation often occurs when statistical strength is treated as evidence of what happened, rather than evidence of whose DNA may be present.

Understanding the distinction between source-level findings and activity-level interpretation is critical when assessing the evidentiary weight of DNA results.

Where likelihood ratios form a central part of the forensic case, independent forensic review can assist in clarifying what those statistics actually mean in context.