# how to calculate type II error $\beta$?

My question is how to calculate type II error $\beta$?

• Suppose I want to test $H_0: \mu=0$ vs $H_1: \mu=1$ (I need to calculate type II error $\beta$, so I need to fix a $\mu$, say 1, in $H_1$).

• Suppose the distribution for $H_0$ is $F_0$, $H_1$ is $F_1$, where $E[\xi]=0$ if $\xi\sim F_0$, $E[\xi]=1$ if $\xi\sim F_1$.

• Now I create an estimator for $\mu$, say $\bar{X}_n$, and a test statistics $S_n=\frac{\bar{X}_n-E[F_0]}{\sigma}=\frac{\bar{X}_n-0}{\sigma}=\frac{\bar{X}_n}{\sigma}$ (let's assume $\sigma$ is known).

• Now I create a reject($H_0$) rule: $S_n>b$.

• Type II error is calculated as $P_{F_1}(S_n>b)$

My questions are(want to verify three things):

• The above construction logic is correct, right?

• The distribution in "$P_{F_1}(S_n>b)$" is $F_1$, right?

• [most care about] The $S_n$ in "$P_{F_1}(S_n>b)$" should use $F_0$ to calculate, right?

• I mean, no matter type I or type II error I am calculate, I always need to use $F_0$ to calculate the test statistics, right?

• I mean, $S_n$ is always $\frac{\bar{X}_n-E[F_0]}{\sigma}$ in type I or type II error calculation, but not $\frac{\bar{X}_n-E[F_1]}{\sigma}$ in calculating $\beta$, right?

• Or, this should not be a problem, because test statistics is just a function of sample and should not involve parameters?

• Type II error is not to reject the null hypothesis when it is false, i.e $H_1$ is true. I think you should use $F_1$ to calculate P but not $F_0$ as you have written $P_{F_1}(S_n>b)$. You also can refer to power calculation whic is based on $H_1$ parameter, and Type II $\beta$ = 1-power Aug 27 '15 at 4:32
• Thank you! You are right. I made a mistake. It is $P_{F_1}(S_n\le b)$ for the type II error. Aug 28 '15 at 13:53

## 1 Answer

Denote $\mathcal{F}^{(0)}(\mu=\mu_0,\sigma=\sigma_0)$ be the distribution under the null hypothesis and $\mathcal{F}^{(1)}(\mu=\mu_1,\sigma=\sigma_1)$ under $H_1$, so you have a test statistic $X$ and you want to test

$H_0: X \sim \mathcal{F}^{(0)}(\mu=0,\sigma=\sigma_0)$ versus $H_1: X \sim \mathcal{F}^{(1)}(\mu=1,\sigma=\sigma_1)$

The way you describe it, you want to perform a one-sided test, and you define the critical region in the right tail. So after you have chosen a confidence level $\alpha$, you will use the distribution $\mathcal{F}^{(0)}(\mu=0,\sigma=\sigma_0)$ to find the quantile value $q_{\alpha}^{(0)}$ such that $P^{(0)}(X \ge q_{\alpha}^{(0)})=\alpha$ ( I am assuming continuous distributions). The superindex $(0)$ indicates that the probabilities are measured under $\mathcal{F}^{(0)}$, so you need the null distribution $\mathcal{F}^{(0)}$ to define the critical region, i.e. the quantile $q_{\alpha}^{(0)}$.

From a sample you can observe an outcome $x$ for the random variable $X$ and the null will be rejected when $x \ge q_{\alpha}^{(0)}$. In other words your test will decide that $H_1 \textrm{ decided as true} \iff x \in [q_{\alpha}^{(0)};+\infty[$.

The power of your test is the probability that $H_1$ is decided as true whenever $H_1$ is true, so the power is the probability that $X \ge q_{\alpha}^{(0)}$ whenever $H_1$ is true, this is the probability that $X \ge q_{\alpha}^{(0)}$ when the true distribution is $\mathcal{F}^{(1)}$ or the power $\mathcal{P}$ is

$\mathcal{P}=P^{(1)}(X \ge q_{\alpha}^{(0)})$

Where the superindex $(1)$ indicates that the probabilities are computed under $\mathcal{F}^{(1)}$ So the power is measured with $\mathcal{F}^{(1)}$ but you need the value of $q_{\alpha}^{(0)}$ which is computed with $\mathcal{F}^{(0)}$.

I used the power $\mathcal{P}$ and the type II error $\beta$ is $\beta=1-\mathcal{P}$.

## In your case

You are right when you say that ''The distribution in "$P_{F_1}(S_n>b)$" is $F_1$''

However, in order to find $b$ you will have to use the $F_0$. In fact, the $b$ is the analog of $q_{\alpha}^{(0)}$