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

Number of heartbeats per minute	Number of women
65 - 68	2
68 - 71	4
71 - 74	3
74 - 77	8
77 - 80	7
80 - 83	4
83 - 86	2

We will use step deviation method.

In the following table a is the assumed mean and h is the class size.

Here, h = 3.

By formula,

Class mark = $\dfrac{\text{Upper limit + Lower limit}}{2}$

By formula,

Mean = a + $\dfrac{Σf$iui}{Σf_i} \times h

Substituting values we get :

$\text{Mean } = 75.5 + \dfrac{4}{30} \times 3 \\[1em] = 75.5 + \dfrac{4}{10} \\[1em] = 75.5 + 0.4 \\[1em] = 75.9$

Hence, mean = 75.9