Sensitivity is the probability of testing positive if you are sick. The probability of a false-negative is 100% minus the sensitivity.
Specificity is the probability of testing negative if you are healthy. The probability of a false-positive is 100% minus the specificity.
The probabaility of you being sick given a test result depends on how prevelant the condition is overall (the base rate). This is confusing at first, but it helps to think in terms of extremes:
- If we know for a fact that 100% of people are sick, then you have a 100% probability of being sick regardless of what any test says.
- If we know for a fact that 0% of people are sick, then you have aa 0% probability of being sick regardless of what any test says.