Z score is a statistic used to measure the number of standard deviations above or below the mean of your population. Therefore to calculate your Z score, you will need to calculate the value of the mean and standard deviation of your sample.
The formula for
Z score = (x- µ ) / σ
µ is the mean of your sample
σ is the standard deviation of your sample
x is single data point in your population.
We will discuss this with an example. Let us say, there are 5 members in your family. Their Age are x (10, 20, 25 15, 20).
The mean age of your family can be calculated as
µ = (10 + 20 + 25 + 15 + 20) /5 = 18
The standard deviation (σ) is to be calculated in a spread sheet
σ = √(Σ (x- µ)^2/N
where N is the population size.
Standard deviation (σ) = 5.099
Let us calculate Z score for age 10
Z score = (10 -18) / 5.099 = – 1.57
o the person with age 10 is 1.57 standard deviation below the average mean i.e 18 of your family.
You may use this formula for calculating the Z score.