# How to calculate p value from z

In the field of statistics, the p value and z score are essential concepts for hypothesis testing. The p value is the probability that the observed results occurred by chance, while the z score measures how far a data point is from the mean in terms of standard deviations. Calculating the p value from a z score is a vital skill for anyone handling statistical data and helps determine the significance of your results. In this article, we’ll walk you through a step-by-step guide on how to calculate the p value from a z score.

**Step 1: Understand your hypothesis**

Before diving into calculations, you need to have a clear understanding of your null and alternate hypotheses. The null hypothesis (H0) typically states that there’s no significant difference between two groups or no effect caused by an intervention, while the alternative hypothesis (H1) claims that there is a significant difference or effect.

**Step 2: Calculate the z score**

To get started with calculating p values, you first need to determine the z score. The formula for calculating z scores is:

Z = (X – μ) / σ

where X is the data point, μ represents the mean of the population or sample, and σ is the standard deviation.

**Step 3: Determine which side of the distribution**

Depending on your hypothesis, you’ll need to decide whether you’re conducting a one-tailed or two-tailed test. In a one-tailed test, you’re only looking for results on one end of the distribution (either greater than OR less than). Conversely, in a two-tailed test, you’re looking for results on both ends of the distribution (both greater than AND less than).

**Step 4: Find the area under the curve using Z table**

You can look up z scores in a standard normal distribution table (also known as Z table) to find the area under the curve. The table shows probabilities for one-tailed tests.

For example, if you have a z score of 1.96, you’ll find a value of approximately 0.975 in the Z table.

**Step 5: Calculate p value**

– If you’re conducting a one-tailed test, the p value can be found directly from the Z table.

For instance, suppose you have a z score of 1.96 and have conducted a one-tailed test.

The area under the curve is approximately 0.975 (from Z table). Thus, your p value would be:

P(Z ≥ 1.96) = 1 – 0.975 = 0.025

– If you’re conducting a two-tailed test, multiply the one-tailed p value by two:

P(Z ≤ -1.96 OR Z ≥ 1.96) = 2 * (0.025) = 0.05

**Step 6: Interpretation**

Finally, compare your calculated p value with your predetermined significance level (α), usually set at 0.05 or 0.01.

– If the p value is less than α, reject the null hypothesis (H0) and conclude that there is sufficient evidence for the alternative hypothesis (H1).

– If the p value is greater than α, do not reject H0; insufficient evidence supports the alternative hypothesis.

By following these steps, you can successfully calculate and interpret p values from z scores and make well-informed conclusions about your statistical data and hypotheses testing.