The following table shows a frequency distribution for grouped data. Notice that the frequency of scores, f(x), does not add up to 15. If a total of 15 scores were actually counted, give a possible explanation for why the frequencies do not add up to 15.
Intervals _____________________ Frequency
0-5 ..............................................4
5-10 .............................................6
10-15 ...........................................3
15-20 ...........................................5