Statistics

GCSE: Maths

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To get a random sample of students I first need to decide how many students from each year and sex I will use. This is easy if I wish to collect for example a random sample of 30 year 7 boys and 30 year 7 girls because I already know how many students to pick at random and from which year and sex. However if I wish to take a random sample from the school as a whole, this is known as a stratified sample, I would need to work out how many males and females to take at random from each year using a formula. This is because different years have different amounts of students and different amounts of males and females within each year, so I need to make sure I take the same percentage of students from each sex in each year to make it a fair test. For example I may need 5% of all the boys in year 7 and 5% of all the girls in year 7 and 5% of all the boys in year 8 etc. but they should all be the same percentage to make it a fair test. To do this I divide the amount of students of one sex in a year group by the amount of students in the whole school and then multiply this by the amount of students needed to be chosen at random.

A x C
B

A – Number of students of one sex in a single year group
B – Number of students in the whole year
C – Number of students to be chosen at random

One example of this is when I did a pretest and required 30 boys and girls from the whole school, and so to do this I used the formula to work out how many students I would need from each year and each sex.

Year 7 boys - 151 x 30 = 3.829247
1183 4 Boys

Year 7 girls - 131 x 30 = 3.322062
1183 3 Girls

Year 8 boys - 145 x 30 = 3.677092
1183 4 boys

etc.
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