How to Calculate Sample Size in Animal Studies Using G* Power Software ?

Three methods were found relevant and preferred in the Literature Survey

1. Selection of sample size from the reference article

2. Calculation of sample size from the Resource Equation Method

3. Calculation of sample size Using Software (G*Power) or Power Calculation Method

We will directly jump on the second point.

II. Resource Equation Method

E = Total number of animals − Total number of groups

• According to this method, a value “E” is measured, which is nothing but the degree of freedom of analysis of variance (ANOVA). The value of E should lie between 10 and 20.

• If E is less than 10, then adding more animals will increase the chance of getting more significant results, but if it is more than 20, then adding more animals will not increase the chance of getting significant results.

• Though, this method is based on ANOVA, it is applicable to all animal experiments.

Any sample size, which keeps E between 10 and 20 should be considered adequate.

Suppose, a researcher wants to see the effect of a drug and he made five groups (one group as control and four groups of different doses of that drug) with 10 rats each. In this case, E will be

E = (10 × 5) − 5

E = 50 − 5 = 45, which is more than 20 hence sample size in this experiment is more than necessary. However, if the sample size is five per group then E will be 20, which is the acceptable limit and hence can be considered an adequate sample size.

Limitation: This method is easy, but it cannot be considered as robust as the power analysis method because it does not consider the factor of effect size.

III. Power Calculation Method Using Software

G Power Software (Faul, Erdfelder, Lang and Buchner, 2007)

• This software can be used for simple as well as complex sample size calculations.

• G Power can calculate sample size based on pre-designed effect size at small, medium, and large differences between the groups based on Cohen’s principles.

• Now we will see how this software works…

Step-1: Software After Switch On..... Shows following interface

Step-2: Selection of Test Family (For more than 2 groups in animal studies select F Tests)

Step-3 : Section of the type of power analysis

Step-4: Selection of Statistical Test: On termination day result measurement, select ANOVA: Fixed effects omnibus, one way for multi-timepoint measurement Select ANOVA: Repeated measure, within-between interaction.

Step-5: Selection of Statistical Significance level 0.05 or 0.01

Step-6: Selection of confidence level 95%, 90 %, or 80% accordingly add 0.95, 0.90, or 0.80 respectively.

Step-7: Addition of number of groups

Step-8: Selection of Effect size from the default setting (0.10 small, 0.25 medium, and 0.40 large)

Step-9: Option for Input parameter (Determine): By using this option we can calculate an effect size from previous data. The below interface shows less sample size, so we can change the power value and recheck.

Step-10: After power value modification, it shows albeit less sample size.

Step-11: So we will go for the default setting, with the medium effect size, i.e. 0.25, again it shows a large sample size.

Step-12: Now change the medium effect size to the large effect size.

Step-13: Check the Power option with an 80 % confidence level. Now it shows 12 animals per group.

Few other free software and calculators are available online for sample size calculation.

This software can be used for sample size calculations through the power analysis method. Some of them can also be used for other statistical methods.

Open Epi http://www.openepi.com/OE2.3/Menu/OpenEpiMenu.htm

EpiTools epidemiological calculators. http://epitools.ausvet.com.au/content.php?page = SampleSize

Java applets for power and sample size http://homepage.stat.uiowa.edu/~rlenth/Power/#Download_ to_run_locally

StatPages http://statpages.org

Biomath http://www.biomath.info/power/index.htm

Department of Biostatistics, Vanderbilt University http://biostat.mc.vanderbilt.edu/twiki/bin/view/Main/ PowerSampleSize

G Power http://www.psycho.uni‑duesseldorf.de/aap/projects/gpower

Power analysis for ANOVA http://www.math.yorku.ca/SCS/Online/power Statistics calculators http://danielsoper.com/statcalc3/default.aspx

References

Faul F, Erdfelder E, Lang AG, Buchner A. G*Power 3: A flexible statistical power analysis program for the social, behavioral, and biomedical sciences. Behav Res Methods 2007;39:175‑91.

Cohen J. Statistical Power Analysis for the Behavioral Sciences. 2nd ed. Hillsdale, NJ: Lawrence Erlbaum; 1988.

Charan J, Kantharia N. How to calculate sample size in animal studies?. Journal of Pharmacology and Pharmacotherapeutics. 2013 Dec;4(4):303-6.

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