What is a Z Test?
Use a Z test when you need to compare group means. Use the 1-sample analysis to determine whether a population mean is different from a hypothesized value. Or use the 2-sample version to determine whether two population means differ.
A Z test is a form of inferential statistics. It uses samples to draw conclusions about populations.
For example, use Z tests to assess the following:
- One sample: Do students in an honors program have an average IQ score different than a hypothesized value of 100?
- Two sample: Do two IQ boosting programs have different mean scores?
In this post, learn about when to use a Z test vs T test. Then we’ll review the Z test’s hypotheses, assumptions, interpretation, and formula. Finally, we’ll use the formula in a worked example.
Z test vs T test
Z tests and t tests are similar. They both assess the means of one or two groups, have similar assumptions, and allow you to draw the same conclusions about population means.
However, there is one critical difference.
In practice, analysts rarely use Z tests because it’s rare that they’ll know the population standard deviation. It’s even rarer that they’ll know it and yet need to assess an unknown population mean!
A Z test is often the first hypothesis test students learn because its results are easier to calculate by hand and it builds on the standard normal distribution that they probably already understand. Additionally, students don’t need to know about the degrees of freedom.
Z and T test results converge as the sample size approaches infinity. Indeed, for sample sizes greater than 30, the differences between the two analyses become small.
William Sealy Gosset developed the t test specifically to account for the additional uncertainty associated with smaller samples. Conversely, Z tests are too sensitive to mean differences in smaller samples and can produce statistically significant results incorrectly (i.e., false positives).
When to use a T Test vs Z Test
Let’s put a button on it.
When you know the population standard deviation, use a Z test.
When you have a sample estimate of the standard deviation, which will be the vast majority of the time, the best statistical practice is to use a t test regardless of the sample size.
However, the difference between the two analyses becomes trivial when the sample size exceeds 30.
Z Test Hypotheses
This analysis uses sample data to evaluate hypotheses that refer to population means (µ). The hypotheses depend on whether you’re assessing one or two samples.
One-Sample Z Test Hypotheses
- Null hypothesis (H0): The population mean equals a hypothesized value (µ = µ0).
- Alternative hypothesis (HA): The population mean DOES NOT equal a hypothesized value (µ ≠ µ0).
When the p-value is less or equal to your significance level (e.g., 0.05), reject the null hypothesis. The difference between your sample mean and the hypothesized value is statistically significant. Your sample data support the notion that the population mean does not equal the hypothesized value.
Two-Sample Z Test Hypotheses
- Null hypothesis (H0): Two population means are equal (µ1 = µ2).
- Alternative hypothesis (HA): Two population means are not equal (µ1 ≠ µ2).
Again, when the p-value is less than or equal to your significance level, reject the null hypothesis. The difference between the two means is statistically significant. Your sample data support the idea that the two population means are different.
These hypotheses are for two-sided analyses. You can use one-sided, directional hypotheses instead. Learn more in my post, One-Tailed and Two-Tailed Hypothesis Tests Explained.
Z Test Assumptions
For reliable results, your data should satisfy the following assumptions:
You have a random sample
Drawing a random sample from your target population helps ensure that the sample represents the population. Representative samples are crucial for accurately inferring population properties. The Z test results won’t be valid if your data do not reflect the population.
Z tests require continuous data. Continuous variables can assume any numeric value, and the scale can be divided meaningfully into smaller increments, such as fractional and decimal values. For example, weight, height, and temperature are continuous.
Other analyses can assess additional data types. For more information, read Comparing Hypothesis Tests for Continuous, Binary, and Count Data.
Your sample data follow a normal distribution, or you have a large sample size
All Z tests assume your data follow a normal distribution. However, due to the central limit theorem, you can ignore this assumption when your sample is large enough.
The following sample size guidelines indicate when normality becomes less of a concern:
- One-Sample: 20 or more observations.
- Two-Sample: At least 15 in each group.
For the two-sample analysis, the groups must contain different sets of items. This analysis compares two distinct samples.
Related post: Independent and Dependent Samples
Population standard deviation is known
As I mention in the Z test vs T test section, use a Z test when you know the population standard deviation. However, when n > 30, the difference between the analyses becomes trivial.
Related post: Standard Deviations
Z Test Formula
These Z test formulas allow you to calculate the test statistic. Use the Z statistic to determine statistical significance by comparing it to the appropriate critical values and use it to find p-values.
The correct formula depends on whether you’re performing a one- or two-sample analysis. Both formulas require sample means (x̅) and sample sizes (n) from your sample. Additionally, you specify the population standard deviation (σ) or variance (σ2), which does not come from your sample.
I present a worked example using the Z test formula at the end of this post.
One Sample Z Test Formula
The one sample Z test formula is a ratio.
The numerator is the difference between your sample mean and a hypothesized value for the population mean (µ0). This value is often a strawman argument that you hope to disprove.
The denominator is the standard error of the mean. It represents the uncertainty in how well the sample mean estimates the population mean.
Learn more about the Standard Error of the Mean.
Two Sample Z Test Formula
The two sample Z test formula is also a ratio.
The numerator is the difference between your two sample means.
The denominator calculates the pooled standard error of the mean by combining both samples. In this Z test formula, enter the population variances (σ2) for each sample.
Z Test Critical Values
As I mentioned in the Z vs T test section, a Z test does not use degrees of freedom. It evaluates Z-scores in the context of the standard normal distribution. Unlike the t-distribution, the standard normal distribution doesn’t change shape as the sample size changes. Consequently, the critical values don’t change with the sample size.
To find the critical value for a Z test, you need to know the significance level and whether it is one- or two-tailed.
|Type of Test
Learn more about Critical Values: Definition, Finding & Calculator.
Z Test Worked Example
Let’s close this post by calculating the results for a Z test by hand!
Suppose we randomly sampled subjects from an honors program. We want to determine whether their mean IQ score differs from the general population. The general population’s IQ scores are defined as having a mean of 100 and a standard deviation of 15.
We’ll determine whether the difference between our sample mean and the hypothesized population mean of 100 is statistically significant.
Specifically, we’ll use a two-tailed analysis with a significance level of 0.05. Looking at the table above, you’ll see that this Z test has critical values of ± 1.960. Our results are statistically significant if our Z statistic is below –1.960 or above +1.960.
The hypotheses are the following:
- Null (H0): µ = 100
- Alternative (HA): µ ≠ 100
Entering Our Results into the Formula
Here are the values from our study that we need to enter into the Z test formula:
- IQ score sample mean (x̅): 107
- Sample size (n): 25
- Hypothesized population mean (µ0): 100
- Population standard deviation (σ): 15
The Z-score is 2.333. This value is greater than the critical value of 1.960, making the results statistically significant. Below is a graphical representation of our Z test results showing how the Z statistic falls within the critical region.
We can reject the null and conclude that the mean IQ score for the population of honors students does not equal 100. Based on the sample mean of 107, we know their mean IQ score is higher.
Now let’s find the p-value. We could use technology to do that, such as an online calculator. However, let’s go old school and use a Z table.
To find the p-value that corresponds to a Z-score from a two-tailed analysis, we need to find the negative value of our Z-score (even when it’s positive) and double it.
In the truncated Z-table below, I highlight the cell corresponding to a Z-score of -2.33.
The cell value of 0.00990 represents the area or probability to the left of the Z-score -2.33. We need to double it to include the area > +2.33 to obtain the p-value for a two-tailed analysis.
P-value = 0.00990 * 2 = 0.0198
That p-value is an approximation because it uses a Z-score of 2.33 rather than 2.333. Using an online calculator, the p-value for our Z test is a more precise 0.0196. This p-value is less than our significance level of 0.05, which reconfirms the statistically significant results.
See my full Z-table, which explains how to use it to solve other types of problems.