Teaching · For Teachers · Research and data

How many to ask
before you send
the survey.

Choose a survey or a two-group comparison and type what you need to detect. The calculator gives the sample size, corrects it for a small population like your own school, and tells you how many to invite once nonresponse is counted.

Plan the sampleWhat you get
$12one-time · every device you sign in on
Buy · $12

Included with Caveat Classroom and Whole-school licenses. Or get it in the For Teachers toolkit, $79 for 72 items.

The sample size calculator

The worked example is made up: a family survey at a K to 8 school of 640 families, aiming for ±5 points. Nothing you type leaves this browser.

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What you get

Teaching · For Teachers$12 · included with Classroom and Whole-school licenses

What you finish with
The responses or students per group needed, the total, the number to invite or enroll after nonresponse or dropout, the steps and a table of alternatives. It prints as a summary and downloads as a spreadsheet.
Who it is for
Teachers and leaders planning surveys or action research, data and evaluation teams, program evaluators and graduate students in education.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
$12 one-time, yours on every device you sign in on; the tool opens once you are signed in with it. Included with Caveat Classroom and Whole-school licenses. Also in the For Teachers toolkit ($79) and the Teaching Suite pass ($199), and 1 more bundle. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Name the study and choose survey or comparison.
  2. Choose the confidence or significance level and expected dropout.
  3. For a survey, type the margin, expected percent and population; for a comparison, the effect size and power.
  4. Read the number to invite or enroll and the table of alternatives.
  5. Print the summary or download the spreadsheet.

Plan the sample

How the size is worked out

Precision you need, divided by the noise you expect.

The worked example: a ±5 point family surveyUnlimited population384.11.96² × 0.5 × 0.5 ÷ 0.05²640 families241384.1 ÷ (1 + 383.1 ÷ 640)40% will not answer402241 ÷ 0.6, rounded upThe worked example: a ±5 point family surveyUnlimited population384.11.96² × 0.5 × 0.5 ÷ 0.05²640 families241384.1 ÷ (1 + 383.1 ÷ 640)40% will not answer402241 ÷ 0.6, rounded up
A ±5 point margin needs about 385 responses from a very large population, but only 241 from 640 families; expecting 40% not to answer, invite 402.

Surveys

The responses needed to estimate a percent within ±e points are z² × p(1 − p) ÷ e², largest when the expected percent is 50, which is why 50 is the safe default. When the population is small and known, such as the families at one school, the finite population correction lowers the number. Halving the margin roughly quadruples the sample.

Comparisons

To detect a difference of d standard deviations between two groups with a two-sided test, each group needs about 2(z for α + z for power)² ÷ d² students. Power is the chance of detecting a real effect of that size. Small effects need large groups: d = 0.2 needs about 393 per group at 80% power, d = 0.5 about 63.

Sources

  1. Cohen, J. (1988). Statistical Power Analysis for the Behavioral Sciences (2nd ed.). Lawrence Erlbaum: power, effect size and sample size.
  2. Cochran, W. G. (1977). Sampling Techniques (3rd ed.). Wiley: sample size for proportions and the finite population correction.
  3. NIST/SEMATECH e-Handbook of Statistical Methods, section 7.2.2.2, sample sizes required (itl.nist.gov).
  4. Institute of Education Sciences. Statistical Power Analysis in Education Research (NCSER 2010-3006): typical effect sizes and planning.

The worked example is made up. Your figures stay in this browser. Support: team@caveatacademic.com.