(Q12). Thirty women were examined in a hospital by a doctor and their of heart beats per minute were recorded and summarised as shown. Find the mean heart beats per minute for these women, choosing a suitable method.

Number of heartbeats/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

Solution :

Number of heartbeats/minute (C.I) Number of women (fi) Class marks (xi) di = xi - a fidi
65-68 2 66.5 -9 -
68-71 4 69.5 -6 -
71-74 3 72.5 -3 -
74-77 8 75.5a 0
77-80 7 78.5 3
80-83 4 81.5 6
83-86 2 84.5 9
- ∑fi = 30 - - ∑fidi =
x̄ = a +
∑fidi
∑fi
 = 75.5 +
30
 = .9