Standard Deviation (OCR AS Biology): Revision Note

15 rats were timed how long it took them to reach the end of a maze puzzle. Their times, in seconds, are given below. Find the mean time.

12, 10, 15, 14, 17,

11, 12, 13, 9, 21,

14, 20, 19, 16, 23

Answer:

Step 1: Calculate the mean

12 + 10 + 15 + 14 + 17 + 11 + 12 + 13 + 9 + 21 + 14 + 20 + 19 + 16 + 23 = 226

226 ÷ 15 = 15.067

Step 2: Round to 3 significant figures

Mean (X̄) = 15.1 seconds

The ear lengths of a population of rabbits were measured.Ear lengths (mm): 62, 60, 59, 61, 60, 58, 59, 60, 57, 56, 59, 58, 60, 59, 57Calculate the mean and standard deviation.

Answer:

Step 1: Calculate the mean

Mean = 885 ÷ 15 = 59 mm

Step 2: Find the difference between each value and the mean

Subtract the mean from each value to find the difference

Example: 62 - 59 = 3

Step 3: Square each difference

Square the difference for each value

Example: 3² = 9

Step 4: Total the differences

Step 5: Divide the total by (n-1) to get value A

36 ÷ (15 - 1) = 36 ÷ 14 = 2.571

Step 6: Get the square root of value A

Standard Deviation = 1.60

Constructing a table like the one above can help you to keep track of all your calculations during the exam!