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Effect Size Calculator FullScreen

You can use this effect size calculator to quickly and easily determine the effect size (Cohen's d) according to the standard deviations and means of pairs of independent groups of the same size.

Effect Size Calculator for T-test

Group 1

Group 2


What is Effect Size Calculator

An effect size calculator is a statistical tool used to estimate the magnitude of an effect or the strength of a relationship between variables. It quantifies the practical significance of a result by measuring the size of the effect beyond mere statistical significance. Effect size measures provide valuable information about the magnitude and meaningfulness of findings in research or experimental studies.

There are various types of effect size measures, and the choice depends on the type of analysis or study design being used. Here are a few commonly used effect size measures:

  1. Cohen's d: This effect size measure is used to quantify the difference between the means of two groups. It is calculated by taking the difference between the means and dividing it by the pooled standard deviation of the groups.

  2. Pearson's r: Also known as the correlation coefficient, Pearson's r measures the strength and direction of the linear relationship between two continuous variables. It ranges from -1 to +1, with 0 indicating no correlation and values closer to -1 or +1 indicating stronger correlations.

  3. Odds Ratio (OR): The odds ratio is commonly used in studies involving binary outcomes or categorical variables. It quantifies the likelihood of an event occurring in one group compared to another group.

  4. Phi coefficient (φ): The phi coefficient is used to measure the association between two dichotomous variables. It is similar to Pearson's r but specifically designed for binary variables.

  5. R-squared (R²): This effect size measure is used in regression analysis to describe the proportion of variance in the dependent variable that can be explained by the independent variables. It ranges from 0 to 1, with higher values indicating a stronger relationship.

To use an effect size calculator, you typically need to input the relevant statistical values such as means, standard deviations, sample sizes, or correlation coefficients. The calculator will then calculate the effect size measure and may provide additional information such as confidence intervals or interpretive guidelines.

Effect size calculators are available online or as part of statistical software packages. They simplify the process of calculating effect sizes, allowing researchers to assess the practical significance of their findings beyond statistical significance.

If you provide me with specific data or information about the analysis you are conducting, I can assist you further by calculating the appropriate effect size measure for your study.

Cohen's d effect size: definition and formula

By effect size, we mean the gap between the mean values of two groups in relation to standard deviation. The size of this gap can be described by effect size regardless of whether a given study design is observational or experimental.

Many publications require the Cohen's d to be reported on the basis that Cohen's means of interpreting the size of an effect, which assists in comprehending the difference between two groups, is generally acknowledged as effective. The Cohen's d statistic is calculated by determining the difference between two mean values and dividing it by the population standard deviation, thus:

Effect Size = (M1 – M2 ) / SD

SD equals standard deviation.

In situations in which there are similar variances, either group's standard deviation may be employed to calculate Cohen's d. If the variances are not similar, the pooled standard deviation should be employed; this comprises the average from the standard deviations for both groups. The pooled standard deviation comprises the root mean square for the two standard deviations and is calculated thus:

SDpooled = √[ (SD12 + SD22) / 2 ]

SD1 equates to the standard deviation for Group 1, with SD2 being the standard deviation for Group 2.

Cohen's d may be employed only with normal data distributions, and the highest levels of accuracy will be obtained when there is equality between the sizes and standard deviations of the groups.

Conventionally, Cohen's d is categorized thus: effect sizes below 0.2 are regarded as small, 0.3-0.5 are regarded as medium, and 0.8+ is regarded as large.

Cohen's d effect sizes should only be regarded as a guideline; effect sizes should be examined within the research context and information from similar studies/interventions may facilitate this evaluation.


Cohen's d = (M1 – M2 ) / SDpooled

Where: M1 and M2 are the means for the 1st and 2nd groups, SDpooled is the pooled standard deviation of the two groups.

To convert Cohen's d into a correlation coefficient (r), use the following equation:

r2 = d2 / (4 + d2)   [Cohen, 1969].

Effect Size Calculator Example

Certainly! To calculate the effect size for a chi-square test of independence, you can use Cramer's V coefficient. Cramer's V measures the strength of the association between two categorical variables. Here's an example of how to calculate the effect size using Cramer's V:

Let's consider the same contingency table we used in the previous example:

              | Lung Cancer   | No Lung Cancer  | Total

Smoker | 70 | 180 | 250 Non-Smoker | 45 | 205 | 250

Total | 115 | 385 | 500

Step 1: Calculate the chi-square statistic.

Assuming we have already calculated the chi-square statistic and found it to be 4.87, as mentioned in the previous example.

Step 2: Determine the minimum value for the number of rows and columns.

In this example, we have 2 rows and 2 columns. The minimum value (k) is the smallest of these two values, which is 2.

Step 3: Calculate the effect size using Cramer's V formula.

Cramer's V is calculated using the formula:

V = sqrt(χ² / (n * (k - 1)))


  • χ² is the chi-square statistic we calculated previously.
  • n is the total sample size.
  • k is the minimum value for the number of rows and columns.

Let's calculate it:

V = sqrt(4.87 / (500 * (2 - 1))) ≈ 0.156

Step 4: Interpret the effect size.

The effect size using Cramer's V ranges from 0 to 1, where 0 indicates no association between variables, and 1 indicates a perfect association. In this example, the effect size is approximately 0.156, which indicates a small association between smoking habits and the occurrence of lung cancer.

Please note that this is just an example of calculating the effect size using Cramer's V for a chi-square test of independence. There are other effect size measures available as well, depending on the specific context and research question.