It's a simple question of statistics. Tests like this are measured according to two metrics:

• Sensitivity- True positive rate, how accurately it detects positive results
• Selectivity - True negative rate, how accurately it identifies negative results.

The false positive rate is just a mathematical equation with these two constant properties of the test, in addition to the percentage of the population that is truly positive.

The more people in a population who are positive for your test, the lower your false positive rate, but the higher the false negative rate (ie. your test will miss people who truly have the disease, this is why many places require you to test negative at least twice on a test before definitively saying you're cured.)

The more people in a population who aren't positive for your test, the lower the false negative rate, but the higher the false positive rate.

This will be true no matter how good your test is, since no test will ever be 100% sensitive and selective.

It's just statistics and probability.

Source: https://www.bmj.com/content/369/bmj.m1808 Very handy simulation for true and false positives.

It's a simple question of statistics. Tests like this are measured according to two metrics:

• Sensitivity- True positive rate, how accurately it detects positive results
• Selectivity - True negative rate, how accurately it identifies negative results.

The false positive rate is just a mathematical equation with these two variables in addition to the percentage of the population that is truly positive.

The more people in a population who are positive for your test, the lower your false positive rate, but the higher the false negative rate (ie. your test will miss people who truly have the disease, this is why many places require you to test negative at least twice on a test before definitively saying you're cured.)

The more people in a population who aren't positive for your test, the lower the false negative rate, but the higher the false positive rate.

This will be true no matter how good your test is, since no test will ever be 100% sensitive and selective.

It's just statistics and probability.

Source: https://www.bmj.com/content/369/bmj.m1808 Very handy simulation for true and false positives.