Belady’s Anomaly in OS

If the number of frames increases, the page fault rate will be decreased.
Example :

7  0  3  0  4  2  3  0  3  2  1  2  0  1  7  0  1 

If we apply the FIFO algorithm with 3 frames, there are 15-page faults that occur, if we increase 4 frames, there are 12-page faults that happened. So here the page fault rate is decreased. This result is most unexpected and it is called Belady’s Anomaly.
Page fault rate = Number of Page faults / Total number of references