But it is very unlikely that the internal response would be 25 spikes per second when no tumor is present. Because I want to remain noncommittal about what and where in the brain the internal response is, I did not label the horizontal axis in terms of firing rates. The internal response is in some unknown, but quantifiable, units. An example criterion is indicated by the vertical lines in Figure 2. False alarms correspond to noise-alone trials when the internal response is greater than criterion, as indicated in the figure.
Vertical lines correspond to the criterion response.
Then they will never miss a tumor when it is present and they will therefore have a very high hit rate. Thus, there is a clear cost to increasing the number of hits, and that cost is paid in terms of false alarms. They will rarely make a false alarm, but they will also miss many real tumors. The message that you should be taking home from this is that it is inevitable that some mistakes will be made. Thus a doctor cannot always be right.