P value of the test
P-value of the test is the probability that the test statistic under null hypothesis will take on values as extreme as or more extreme than the observed value of test statistic.
The smaller p-value of the test indicate strong evidence against the null hypothesis $H_0$.
In this tutorial we discuss about how to find p value from Z score. The p-value of the Z score depends on the direction of the alternative hypothesis.
p-Value for Z-score
If the test statistic $Z$ has standard normal distribution, then the $p$-value of the test for testing
a. left-tailed hypothesis is $p$-value = $P(Z\leq z_{obs})$.
b. right-tailed hypothesis is $p$-value = $P(Z\geq z_{obs})$.
c. two-tailed hypothesis is $p$-value = $2P(Z\geq |Z_{obs}|)$.
Interpretation from p-value
If the $p$-value of the test is less than or equal to the level of significance ($\alpha$) (i.e., $p \leq \alpha$), we reject the null hypothesis $H_0$ at $\alpha$ level of significance.
If the $p$-value of the test is greater than the level of significance ($\alpha$) (i.e., $p > \alpha$), we fail to reject the null hypothesis $H_0$ at $\alpha$ level of significance.
P value from Z Score Calculator
Use this calculator to compute the P value from Z score.
| P Value from Z score Calculator | ||
|---|---|---|
| Z score : ($z$) | ||
| Tail : | Left tailedRight tailedTwo tailed | |
| Results | ||
| P-value: | ||
How to find P Value from Z Score calculator?
Step 1 - Enter the value of Z score value.
Step 2 - Select the alternative hypothesis (i.e. Left-tailed / Right-tailed / Two-tailed)
Step 3 - Click on "Calculate" button to get the p-value from z score.
Example 1 - Find p value for right tailed test from Z score
Find the P value for a right-tailed hypothesis testing problem with a z test statistic $Z_{obs}=2.65$. What is your decision if the level of significance is $\alpha = 0.05$?
Solution
The test statistic is $Z_{obs}=2.65$. The alternative hypothesis is right-tailed.
Because the alternative hypothesis is right-tailed, the $p$-value of the test is given by
$$ \begin{aligned} p&=P(Z\geq Z_{obs})\\ &=P(Z\geq 2.65)\\ &= 0.004 \end{aligned} $$
Because the p-value ($0.004$) is $\text{less than}$ the significance level $\alpha = 0.05$, we $\text{reject}$ the null hypothesis at $0.05$ level of significance.
Example 2 - Find p value for left tailed test from Z score
Find the P-value for a left-tailed hypothesis testing problem with a z test statistic $Z_{obs}=-2.023$. What is your decision if the level of significance is $\alpha = 0.01$?
Solution
The z test statistic is $Z_{obs}=-2.023$. The alternative hypothesis is left-tailed.
Because the alternative hypothesis is left-tailed, the P-value of the test is given by
$$ \begin{aligned} p&=P(Z\leq Z_{obs})\\ &=P(Z\leq -2.023)\\ &= 0.0215 \end{aligned} $$
Because the p-value ($0.0215$) is $\text{greater than}$ the significance level $\alpha = 0.01$, we $\text{fail to reject}$ the null hypothesis at $0.01$ level of significance.
Example 3 - Find p-value for two tailed test from z-score
Find the p value for a two-tailed hypothesis testing problem with a z test statistic $Z_{obs}=1.674$. What is your decision if the level of significance is $\alpha = 0.05$.
Solution
The test statistic is $Z_{obs}=1.674$. The alternative hypothesis is two-tailed.
Because the alternative hypothesis is two-tailed, the P-value of the test is given by
$$ \begin{aligned} p&=2*P(Z\geq |Z_{obs}|)\\ &=2*P(Z\geq |1.674|)\\ &=2*0.0471\\ &= 0.0942 \end{aligned} $$
Because the p-value ($0.0942$) is $\text{greater than}$ the significance level $\alpha = 0.05$, we $\text{fail to reject}$ the null hypothesis at $0.05$ level of significance.
Z-tests calculators
Here you will find some easy to use statistics calculators with illustrated examples on Z-tests.
- Z-test for single proportion
- Z-test for single mean
- Z-test for two proportions
- Z-test for two means
- Z-test for testing correlation coefficient
- Z-test for testing homogeneity of two correlation coefficients
Conclusion
In this tutorial, you learned about how to find the p-value from z-score. You also learned about how to solve numerical problems based on p-value for z-test.
To learn more about other parametric hypothesis testing, please refer to the following tutorials:
Let me know in the comments if you have any questions on p-value calculator for z-test and your thought on this article.
