Question

A lab found that 670 rats could run through a maze in a mean time of 4.7 seconds. What is the 99% confidence interval for the population mean? Use the formula for margin of error: z•σ/√n. Please explain each step, particularly how to find the population standard deviation.

Answer 1

Using the formula for margin of error as

MOE = z * σ / √n

would be very difficult since we are not given the value of the standard deviation. Standard deviation value must be given since it is obtained from the experiment.

However, we use another formula for MOE in the form of:

MOE = z sqrt [p (1 – p) / n]

where p is the proportion at 99% confidence interval at z crit value. From the standard distribution tables, this corresponds to a p value of:

z crit = 2.58

p = 0.9951

Therefore the margin of error is:

MOE = 2.58 sqrt [0.9951 (1 – 0.9951) / 670]

MOE = 6.96 x 10^-3 = 0.00696 s

 

We can see that at 99% Confidence interval, the Margin of Error is extremely small (almost 0). For the sake of calculation:

Confidence interval = 4.7 s ± 0.00696 s

Confidence interval = 4.69304, 4.70696