# Z Critical Value Calculator

What type of test?
Probability Value (α)
Critical Value  =  -0.85
Critical Region  =  [-∞, -0.85)
GENERATE WORK

## Z Critical Value Calculator

Z Critical Value Calculator uses a significance level value and test (left-tailed, right-tailed, two-tailed) to calculate z-critical value for the standard normal distribution N(0,1), or close to normal distribution.
The Z Critical Value Calculator uses two inputs to provide the the critical values for a standard normal distribution. It is necessary to follow the next steps:
• Enter the significance level (the α value). The value must be in the range [0,1]. The significance level, α, is the probability of rejecting the null hypothesis when it is true;
• Choose one of the following tests: left-tailed, right-tailed, two-tailed;
• Press the ”Generate Work” button to make the computation;
Z-critical Value Formula:
The z-critical value can be calculated by the following formula
• left-tailed test: Φ-1(α);
• right-tailed test: Φ-1(1 − α);
• two-tailed test: ±Φ-1(1 - α/2)
where Φ is the cumulative distribution function of the standard normal distribution.

## What is a z-critical value?

In statistics, finding critical values is a method that allows us to decide whether to retain or reject the null hypothesis. The critical value determines if the value of the test statistic formed by given sample belongs to the rejection region. Z-critical value is used when the population standard deviation is known or for larger sample sizes. Z-critical value of a one-tailed test is a limit value that constitutes the boundary of the rejection region. In the case of a two-tailed test, we have two limit values.
First, we need to set a significance level, α, which determines the probability of rejecting the null hypothesis when it is correct. Common significance levels most often use are 0.05 (95% confidence), 0.025 (97.5%), and 0.01 (99%). Note also that the definition of z-critical values is “backwards” from that of percentiles. For instance, z0.03 is the 97th percentile of the standard normal distribution.

### How to calculate z-critical value?

To determine the z-critical value, the distribution of hypothesis testing must be the standard normal distribution. The standard normal distribution is the normal distribution with mean 0 and standard deviation 1. Z-critical values are the points on the distribution which have the same probability as our test statistic, equal to the significance level α.
If the test is one-sided, then there is only one critical value, if it is two-sided, then there are two critical values, one to the left and the other to the right of the median value of the distribution.
The formulae for the z-critical values involve the quantile function, Φ-1(z), which is the inverse of the cumulative distribution function of the standard normal distribution. The cumulative distribution function of the standard normal distribution is denoted by the formula
Φ(z) = P (Z ≤ z) =   1  \sqrt{2π} x−∞ e-u2/2 du
Therefore, we need to find the inverse of the function Φ(z). The notation is to find zα that α (which is between 0 and 1) is the probability that Z > zα, i.e.

P (Z > zα) = 1 − Φ (zα) = α
In other words, zα is the value of z after which the area under the standard normal distribution is α. Thus, area before zα is 1−α. For finding z-critical values by hand, we need to use the table of Φ(z) values. Some known values are:

z0 = +∞,     z0.5 = 0,     z1 = −∞
For example, let us find z0.25 is the z that the area before that is 0.75. So, in the table of Φ(z) have to find the z related to 0.75.

We have the following options for selecting the test:
• In a left-tailed test, the area under the density curve from the critical value to the left is equal to α. In this case, the z-critical value can be calculated as Φ-1(α);

• In a right-tailed test, the area under the density curve from the critical value to the right is equal to α. In this case, the z-critical value can be calculated as Φ-1(1 − α);

• In a two-tailed test, the area under the density curve from the left critical value to the left is α/2 and the area under the curve from the right critical value to the right is α/2. In this case, the z-critical value can be calculated as ±Φ-1(1 - α/2)

Z Critical Value Calculator is important for conducting statistical research.