The number of telephone calls received at an exchange per interval for 250 successive one-minute intervals are given in the following frequency table.


No. of calls (x):



0



1



2



3



4



5



6



No. of intervals (f):



15



24



29



46



54



43



39



Compute the mean number of calls per interval.

Let the assumed mean (A) = 3


Number of calls (xi)



Number of intervals (fi �)



ui = xi – A = xi - 3



fiui



0



15



-3



-45



1



24



-2



-48



2



29



-1



-29



3



46



0



0



4



54



1



54



5



43



2



86



6



39



3



117




N = 250




fiui = 135



Mean number of calls = A +


= 3 + =


= 3.54


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