# To find out concentration of $$SO_2$$ in the air (in parts per million, i.e. ppm), the data was collected for 30 localities in a certain city and is presented below. Find the mean concentration of $$SO_2$$ in the air.

## Solution :

Let the assumed mean a = 0.10

By step deviation method :

Mean = a + $$\sum f_iu_i\over \sum f_i$$ $$\times$$ h = 0.10 + $$-1\over 30$$ $$\times$$ 0.04

= 0.10 – 0.0013 = 0.0987

= 0.099 ppm