One can argue that the evidential value of a database hit increases when the database gets larger. Indeed, having a unique hit in a database simply means that all the other persons in the database are innocent. Therefore, if the database is larger we have the extra information that many more people are innocent. Clearly, this increases the weight of the evidence against the suspect.
- Is this reasoning correct?
- Is the probability of a unique match the same, irrespective of the size of the dataset?
This is how I see it: if the database contains everybody, then a unique hits tells us with certainty who the donor of the DNA prole is. So it really is true that a larger database is stronger evidence. But then I started thinking: suppose the database consists of 16,000,000 proles, and suppose that the DNA prole we are looking for has frequency 1/1,000,000 (one in a million). Is it still reasonable to believe that the underlying model is correct?