Suppose we have a null hypothesis H_{0 }and an alternative hypothesis H_{1}. We consider the distribution given by the null hypothesis and perform a test to determine whether or not the null hypothesis should be rejected in favour of the alternative hypothesis.

There are two different types of tests that can be performed. A **one-tailed** test looks for an increase or decrease in the parameter whereas a **two-tailed** test looks for any change in the parameter (which can be any change- increase or decrease).

We can perform the test at any level (usually 1%, 5% or 10%). For example, performing the test at a 5% level means that there is a 5% chance of wrongly rejecting H_{0 }.

If we perform the test at the 5% level and decide to reject the null hypothesis, we say “there is significant evidence at the 5% level to suggest the hypothesis is false”.

__One-Tailed Test__

We choose a critical region. In a one-tailed test, the critical region will have just one part (the red area below). If our sample value lies in this region, we reject the null hypothesis in favour of the alternative.

Suppose we are looking for a definite decrease. Then the critical region will be to the left. Note, however, that in the one-tailed test the value of the parameter can be as high as you like.

**Example**

Suppose we are given that X has a Poisson distribution and we want to carry out a hypothesis test on the mean, λ, based upon a sample observation of 3.

Suppose the hypotheses are:

H_{0 }: λ= 9

H_{1 }: λ < 9

We want to test if it is “reasonable” for the observed value of 3 to have come from a Poisson distribution with parameter 9. So what is the probability that a value as low as 3 has come from a Po(9)?

P(X ≤ 3) = 0.0212 (this has come from a Poisson table)

The probability is less than 0.05, so there is less than a 5% chance that the value has come from a Poisson(3) distribution. We therefore reject the null hypothesis in favour of the alternative at the 5% level.

However, the probability is greater than 0.01, so we would not reject the null hypothesis in favour of the alternative at the 1% level.

**Two-Tailed Test**

In a two-tailed test, we are looking for either an increase or a decrease. So, for example, H_{0 } might be that the mean is equal to 9 (as before). This time, however,H_{1 }would be that the mean is not equal to 9. In this case, therefore, the critical region has two parts :