What Is A Good Sample Size For Quantitative Research

1 of the most common questions I go asked by people doing surveys in international evolution is "how big should my sample size be?". While at that place are many sample size calculators and statistical guides bachelor, those who never did statistics at university (or have forgotten it all) may find them intimidating or hard to employ.

If this sounds similar you lot, then keep reading. This guide will explain how to choose a sample size for a bones survey without any of the complicated formulas. For more like shooting fish in a barrel rules of pollex regarding sample sizes for other situations, I highly recommend Sample size: A crude guide by Ronán Conroy and The Survey Enquiry Handbook by Pamela Alreck and Robert Settle.

This article is a brusque introduction to the topic for a more in-depth coverage of the topic consider enrolling in the gratuitous online grade offered by Academy of Florida.

This advice is for:

Basic surveys such as feedback forms, needs assessments, opinion surveys, etc. conducted equally part of a program.
Surveys that use random sampling.

This communication is Non for:

Research studies conducted by universities, research firms, etc.
Complex or very large surveys, such as national household surveys.
Surveys to compare between an intervention and control group or before and later a plan (for this situation Sample size: A rough guide).
Surveys that apply non-random sampling, or a special blazon of sampling such as cluster or stratified sampling (for these situations see Sample size: A rough guide and the United nations guidelines on household surveys).
Surveys where you lot plan to use fancy statistics to analyse the results, such as multivariate analysis (if you know how to do such fancy statistics and so y'all should already know how to choose a sample size).

The minimum sample size is 100

Nearly statisticians agree that the minimum sample size to get whatever kind of meaningful result is 100. If your population is less than 100 and so yous actually demand to survey all of them.

A proficient maximum sample size is usually 10% every bit long every bit information technology does not exceed m

A practiced maximum sample size is usually around 10% of the population, every bit long equally this does not exceed thou. For example, in a population of 5000, 10% would exist 500. In a population of 200,000, 10% would be 20,000. This exceeds 1000, so in this case the maximum would be 1000.

Even in a population of 200,000, sampling chiliad people will normally give a fairly accurate result. Sampling more 1000 people won't add much to the accuracy given the actress time and money it would cost.

Choose a number between the minimum and maximum depending on the situation

Suppose that yous desire to survey students at a schoolhouse which has 6000 pupils enrolled. The minimum sample would be 100. This would give you a rough, but still useful, idea near their opinions. The maximum sample would be 600, which would give y'all a adequately accurate idea about their opinions.

Choose a number closer to theminimum if:

Y'all have limited time and money.
You only need a rough gauge of the results.
Y'all don't programme to divide the sample into different groups during the analysis, or you only plan to use a few large subgroups (e.thousand. males / females).
You think most people will requite similar answers.
The decisions that will be made based on the results do not have significant consequences.

Choose a number closer to themaximum if:

You have the time and money to do information technology.
It is very important to get accurate results.
You plan to divide the sample into many dissimilar groups during the analysis (eastward.g. dissimilar age groups, socio-economical levels, etc).
You call up people are probable to give very different answers.
The decisions that will exist made based on the results of the survey are important, expensive or have serious consequences.

In practice most people usually want the results to be as accurate as possible, so the limiting factor is usually time and coin. In the example above, if you had the fourth dimension and money to survey all 600 students and then that volition give y'all a fairly authentic result. If you don't take enough time or coin so just choose the largest number that you tin can manage, as long as it'south more than 100.

If you lot would similar to learn more about Survey Data Collection consider taking the free course offered by University of Michigan and University of Maryland. Enroll here.

If yous want to be a fleck more scientific then use this tabular array

While the previous rules of thumb are perfectly acceptable for most basic surveys, sometimes yous need to audio more "scientific" in order to exist taken seriously. In that case you can use the following table. Simply cull the column that nearly closely matches your population size. Then choose the row that matches the level of error yous're willing to accept in the results.

Sample size table

You lot volition see on this table that the smallest samples are all the same around 100, and the biggest sample (for a population of more than 5000) is still around 1000. The aforementioned general principles employ as earlier – if you programme to carve up the results into lots of sub-groups, or the decisions to be fabricated are very of import, you should pick a bigger sample.

Note: This table can but be used for basic surveys to measure what proportion of the population have a item feature (e.g. what proportion of farmers are using fertiliser, what proportion of women believe myths about family unit planning, etc). Information technology can't be used if you are trying to compare two groups (e.g. control versus intervention) or two points in time (e.g. baseline and endline surveys). See Sample size: A rough guide for other tables that can be used in these cases.

Relax and stop worrying almost the formulas

It's a dirty piffling secret amid statisticians that sample size formulas ofttimes crave you to have data in accelerate that you don't normally take. For example, you typically need to know (in numerical terms) how much the answers in the survey are likely to vary between individuals (if y'all knew that in advance then you wouldn't be doing a survey!).

Then even though it's theoretically possible to calculate a sample size using a formula, in many cases experts still finish upward relying rules of thumb plus a practiced deal of common sense and pragmatism. That means you lot shouldn't worry too much if you lot can't use fancy maths to choose your sample size – you're in skilful visitor.

One time you lot've called a sample size, don't forget to write good survey questions,design the survey form properly and pre-test and airplane pilot your questionnaire.

Photo past James Cridland