If you take a normally distributed variable with expectation and standard deviation and use normal probability plotting paper to plot the probabilities against g, then you will get a straight line: on normal probability plotting paper, every true normal distribution will become a straight line.
If you transform your given frequency distribution into a cumulative frequency distribution
and plot the values on normal probability plotting paper, then you should get a straight
line if the frequencies are distributed normally. Make sure to plot the cumulative
relative frequencies against the upper bounds of the classes!
Use the following link for a sheet of normal probability plotting paper:
Normal probability
plotting paper
You often find that the points of the cumulative relative frequency distribution do
not precisely lie on a straight line on normal probability plotting paper. You then
draw a line that fits the points as closely as possible. In effect you approximate
your frequency distribution by a normal distribution corresponding to the line.
You can then find an estimate for the expectation by reading off the value corresponding
to 50% on the line. Also, since one of the rules of thumb states that in a normal
distribution 68% of observations are found in the interval , you can use the 84% point of the line to determine the value of .