In a large company the time taken for an employee to carry out a certain task is a normally distributed Random variable with mean $78.0s$ and unknown variance. A new training scheme is introduced and after its introduction the times taken by a random sample of $120$ employees are recorded. The mean time for the sample is $76.4s$ and an unbiased estimate of the population variance is $68.9s$.

(i) Test, at the $1$% significance level, whether the mean time taken for the task has changed.

(ii) It is required to redesign the test so that the probability of making a Type I error is less than $0.01$ when the sample mean is $77.0s$. Calculate an estimate of the smallest sample size needed, and explain why your answer is only an estimate.

This question is taken from a past A level stats $2$ paper. I'm having trouble understanding part ii) of this (part i included in case it's relevant). The mark scheme says to standardise $1$ with $n$ and $2.576$, which I understand is the critical level of the test, but I really don't understand what's going on here so any explanation would be greatly appreciated!


2 Answers 2


The problem, as posted seems flawed in several ways, so I will answer step by step.

First, the units of an unbiased estimate of $\sigma^2$ must be in squared units, here $S^2 = 68.9sec{}^2.$ Then the (very slightly biased) estimate of $\sigma$ is $S = 8.3s.$

In (i) it seems that you must assume that the historical population mean is $\mu_0 = 78s.$ Presumably, this is based on a large enough sample that this population mean for the old training scheme is known with negligible error.

Then, to test $H_0: \mu = 78$ against $H_1: \mu \ne 78,$ based on a sample of size $n = 120$ with sample mean $\bar X = 76.4$ and $S = 8.3,$ one would obtain the test statistic $$T =\frac{76.4 - 78}{8.3/\sqrt{120}} = -2.111701.$$ Under $H_0,$ the test statistic has Student's t distribution with 119 degrees of freedom. The value that cuts probability $0.005$ from the upper tail of this distribution is $t^* = 2.618$ (sometimes sloppily approximated by a standard normal distribution as $t^* \approx 2.576).$ Because $|T| = 2.112 < 2.618$ (or 2.576, if you insist), $H_0$ is not rejected at the 1% level of significance, but is rejected at the 5% level. [The advice to approximate t by z when $n > 30$ is reasonably good for tests at the 5% level, but not so good at the 1% level--as we have just seen.]

The P-value 0.0368 of this two-sided t test is the sum of the areas under the density curve of $\mathsf{T}(df = 119)$ to the left of -2.112 and above 2.112.

Relevant computations in R statistical software for some values given above are shown below:

qt(.005, 119)
[1] -2.617776
[1] -2.575829
pt(-2.112, 119)*2
[1] 0.03677776

Relevant output from Minitab statistical software is as follows:

One-Sample T 

Test of μ = 78 vs ≠ 78

  N    Mean  StDev  SE Mean       99% CI           T      P
120  76.400  8.300    0.758  (74.417, 78.383)  -2.11  0.037

For (ii) it seems that someone wants to increase the sample size so that if $\bar X = 77$ for the larger sample, then one would (just barely) reject $H_0$ against the two-sided alternative. The unwarranted assumption here is that $\sigma$ for the population trained under the new scheme would match the sample standard deviation $S = 8.3$ of the current experiment. In that case, we would have a z test with test statistic $|Z| = \frac{|77 - 78|}{8.3/\sqrt{n}} = 2.576.$ So $n = 458$ should suffice.

In a real application, a more worthwhile problem would be to find the sample size required to detect, with probability $95\%,$ a mean difference of 1 second if $\sigma = 8.3$ in a test of size $\alpha = 1\%$ or $5\%.$ That is, we seek $n$ that would give power 0.95 under such conditions. For a test at the 5% level, the Minitab output below answers this question as $n = 898,$ using an exact computation based on t distributions. [This method requires use of non-contral t distributions; for moderately large sample sizes, a useful approximation can be found using the standard normal distribution.] For a test at the 1% level the answer is $n = 1231$ (output not shown).

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Note: Because the company likely wants to reduce the time it takes employees to do the task under discussion, it seems to me that the appropriate hypotheses would be $H_0: \mu = 78$ vs. $H_a: \mu < 78,$ but the statement of the problem clearly calls for a two-sided alternative. If you are studying for A level exams, you might try working the one-sided version on your own. Maybe the exam you take will have been prepared with more attention to practical concerns.


For part (ii), you are being asked to provide the minimum sample size where a difference of only one in the mean is significant at the $99\%$ level. The probability of making a type I error is $1\%$ at the $99\%$ level so this is just an indirect way of saying "the $99\%$ level". In other regards it's kind of backwards because you don't know the mean you are dealing with until you've calculated it with a known $n$.

For both $t$ and $Z$ equations, the only difference is in the standard deviation, sample versus population. $$t(Z) = \frac{\bar x - \mu_0}{s(\sigma)/\sqrt{n}}=\frac{1\sqrt n}{\sqrt{68.9}} = \frac{\sqrt n}{8.3}$$

Looking at a $t$ table, the lowest value in the column at the $99\%$ significance level is $2.576$ to which they reference a $Z$ for the $df$, meaning $Z = 2.576$ at the $99\%$ level. $$2.576 = \frac{\sqrt n}{8.3}$$ $$n = (2.576\cdot 8.3)^2 = 457.14$$

The reason this is only an estimate is because we used the sample standard deviation instead of the population standard deviation for a $Z$ score calculation of n.


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