Thirty women were examined in a hospital by a doctor and the number of heart beats per minute were recorded and summarised as follows. Find the mean heart beats per minute for these women, choosing a suitable method


Number of heart beats per minute



65 – 68



68 - 71



71 - 74



74 - 77



77 - 80



80 - 83



83 - 86



Number of women



2



4



3



8



7



4



2



The above given data can be represented in the form of table as below:


Class Interval



fi



xi



di = xi - a



fidi



65-68



2



66.5



-9



-18



68-71



4



69.5



-6



-24



71-74



3



72.5



-3



-9



74-77



8



75.5



0



0



77-80



7



78.5



3



21



80-83



4



81.5



6



24



83-86



2



84.5



9



18




∑fi = 30





∑fidi = 12



Now, mean of the deviation can be calculated as follows:




= 0.4


Mean can be calculated as follows:


=


= 0.4 + 75.5


= 75.9


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