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WHAT TEST?

STATISTICAL TESTS FOR SIGNIFICANCE

What test do I need?

Other parts of this site explain how to do the common statistical tests. Here is a guide to choosing the right test for your purposes. When you have found it, click on "more information?" to confirm that the test is suitable. If you know it is suitable, click on "go for it!"

Important: Your data might not be in a suitable form (e.g. percentages, proportions) for the test you need. You can overcome this by using a simple transformation. Always check this - click HERE.

1. Student's t-test

Use this test for comparing the means of two samples (but see test 2 below), even if they have different numbers of replicates. For example, you might want to compare the growth (biomass, etc.) of two populations of bacteria or plants, the yield of a crop with or without fertiliser treatment, the optical density of samples taken from each of two types of solution, etc. This test is used for "measurement data" that are continuously variable (with no fixed limits), not for counts of 1, 2, 3 etc. You would need to transform percentages and proportions because these have fixed limits (0-100, or 0-1).

2. Paired-samples test

Use this test like the t-test but in special circumstances - when you can arrange the two sets of replicate data in pairs. For example: (1) in a crop trial, use the "plus" and "minus" nitrogen crops on one farm as a pair, the "plus" and "minus" nitrogen crops on a second farm as a pair, and so on; (2) in a drug trial where a drug treatment is compared with a placebo (no treatment), one pair might be 20-year-old Caucasian males, another pair might be 30-year old Asian females, and so on.

3. Analysis of variance for comparing the means of three or more samples

Use this test if you want to compare several treatments. For example, the growth of one bacterium at different temperatures, the effects of several drugs or antibiotics, the sizes of several types of plant (or animals' teeth, etc.). You can also compare two things simultaneously - for example, the growth of 3 bacteria at different temperatures, and so on. Like the t-test, this test is used for "measurement data" that are continuously variable (with no fixed limits), not for counts of 1, 2, 3 etc. You would need to transform percentages and proportions because these have fixed limits (0-100, or 0-1).

More information? You need this, because there are different forms of this test.

4. Chi-squared test for categories of data

Use this test to compare counts (numbers) of things that fall into different categories. For example, the numbers of blue-eyed and brown-eyed people in a class, or the numbers of progeny (AA, Aa, aa) from a genetic crossing experiment. You can also use the test for combinations of factors (e.g. the incidence of blue/brown eyes in people with light/dark hair, or the numbers of oak and birch trees with or without a particular type of toadstool beneath them on different soil types, etc.).

5. Poisson distribution for count data

Use this test for putting confidence limits on the mean of counts of random events, so that different count means can be compared for statistical difference. For example, numbers of bacteria counted in the different squares of a counting chamber (haemocytometer) should follow a random distribution, unless the bacteria attract one another (in which case the numbers in some squares should be abnormally high, and abnormally low in other squares) or repel one another (in which case the counts should be abnormally similar in all squares). Very few things in nature are randomly distributed, but testing the recorded data against the expectation of the Poisson distribution would show this. By using the Poisson distribution you have a powerful test for analysing whether objects/ events are randomly distributed in space and time (or, conversely, whether the objects/ events are clustered).

6. Correlation coefficient and regression analysis for curve fitting

These procedures are used for looking at the relationship between different factors, and (if appropriate) for graphing the results in statistically meaningful ways. For example, as the temperature (or pH, etc.) increases, does growth rate increase or decrease? As the dose rate of a drug is increased does the response rate of patients rise? As altitude is increased does the number of butterflies (or oak trees) increase or decrease? Sometimes the relationship is linear, sometimes logarithmic, sometimes sigmoidal, etc. You can test all these possibilities and, in drug or toxicity trials (for example) calculate the LD50 or ED50 (lethal dose, or estimated dose, for a 50% response rate).

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Student's t-test

Use this test for comparing the means of two populations that you have sampled (but see test 2 below). For example, you might want to compare the growth (biomass, etc.) of two bacteria or plants, the yield of a crop with or without added nitrogen, the optical density of samples taken from each of two types of solution, etc.

What you will need for this test: a minimum of 2 or 3 replicates of each sample or treatment, but ideally at least 5 replicates. For example, the yield measured for 5 fields of a crop fertilised with nitrogen and for 5 unfertilised fields, the optical density of 5 tubes of each solution, the measurement of 5 plants of each type, etc. Large sample sizes (10 or more) are always better than small sample sizes, but it is easier to measure the height of 10 or 20 (or 50) plants than it is to set up10 or 20 large-scale fermenters!

You don't need the same number of replicates of each treatment - for example, you can compare 3 tubes of one solution with 4 tubes of another. You could also use this test to compare several replicates of one treatment with a single value for another treatment, but it would not be very sensitive.

Paired-samples test

Use this test like the t-test but in special circumstances - when you can arrange the two sets of replicate data in pairs. For example: (1) in a crop trial, use the "plus" and "minus" nitrogen crops on one farm as a pair, the "plus" and "minus" nitrogen crops on a second farm as a pair, and so on; (2) in a drug trial where a drug treatment is compared with a placebo (no treatment), one pair might be 20-year-old males, another pair might be 30-year old females, and so on.

Why do we use the paired samples test? Because farms or people or many other things are inherently variable, but by pairing the treatments we can remove much of this random variability from the test of "nitrogen versus no nitrogen" or "drug treatment versus no treatment", etc.

What are the requirements for this test? The main requirement is that the experiment is PLANNED ahead of time. Then you can use the paired samples test for many purposes - for example, two treatments compared on one day, then the same two treatments compared on the next day, and so on.

In general, you will need more replicates than for a t-test (say, a minimum of 5 for each treatment), and you will need the same number of replicates for each treatment.

But you must have a good reason to pair treatments - you should not do it arbitrarily.

Analysis of variance for comparing the means of three or more samples.

Use this test if you want to compare several treatments. For example, the growth of one bacterium at different temperatures, the effects of several drugs or antibiotics, the sizes of several plants (or animals' teeth, etc.). You can also compare two things simultaneously - for example, the growth of 3 or 4 strains of bacteria at different temperatures, and so on.

The simplest form of this test is one-way ANOVA (ANalysis Of VAriance). Use this to compare several separate treatments (e.g. effects of 3 or more temperatures, antibiotic levels, crop treatments, etc.). You will need at least 2 replicates of each treatment.

One-way ANOVA tells you if there are differences between the treatments as a whole. But it can also be used, with caution, like a multiple t-test, to tell you which of the treatments differ from each other.
Go for one-way ANOVA?
Back to "What test do I need?"

Another form of this test is two-way ANOVA. Use this if you want to compare combinations of treatments. For example, to compare the growth of an organism on several different substrates at several different temperatures. Or the effects of two (or more) drugs singly and in combination. Or responses of crops to fertiliser treatment on different farms or soil types. You can get useful information even if you have one of each combination of treatments, but you get much more information if you have 2 (or more) replicates of each combination of treatments. Then the test can tell you if you have significant interaction - for example, if changing the temperature changes the way that an organism responds to a change of pH, etc.
Go for two-way ANOVA?
Back to "What test do I need?"

Chi-squared test for categories of data

Use this test to compare counts (numbers) of things that fall into different categories. For example, to compare the numbers of blue-eyed and brown-eyed people in a class, or the numbers of progeny (AA, Aa, aa) from a genetic crossing experiment. You can also use the test for looking at combinations of factors (e.g. the incidence of blue/brown eyes in people with light/dark hair, or the numbers of toadstools beneath oak and birch trees on different soil types, etc.).

For this test you compare the actual counts (in the different categories) with an "expected" set of counts. Sometimes the expectation is obvious - for example, that half of the progeny from a cross between parents Aa and aa will have the Aa genotype and half will have aa. You have to construct an hypothesis (termed the null hypothesis) by using logical arguments.

What are the requirements for this test? Almost any sort of "count" data can be analysed by chi-squared, but you have to use "real" numbers, not proportions or percentages.

Poisson distribution for count data

The main requirement for this test is that the mean count (of bacterial colonies, buttercups, etc.) need to be relatively high (say 30 or more) before they can be expected to conform to a Poisson distribution. If you have such a high count, then you can test whether or not your results actually do conform to the Poisson distribution.

Correlation coefficient and regression analysis for curve fitting

These procedures are used for looking at the relationship between different factors, and (if appropriate) for graphing the results in statistically meaningful ways. For example, as the temperature (or pH, etc.) increases, does growth rate increase or decrease? As the dose rate of a drug is increased does the response rate of patients rise? As altitude is increased does the number of butterflies (or oak trees) increase or decrease? Sometimes the relationship is linear, sometimes logarithmic, sometimes sigmoidal, etc. You can test all these possibilities and, in drug or toxicity trials (for example) calculate the LD50 or ED50 (lethal dose, or estimated dose, for a 50% response rate).

There is a 3-stage procedure:

1. Plot your results on graph paper, and ask yourself: does the relationship look (or is expected to be) linear, or is it logarithmic, or sigmoid (S-shaped)? You might need to transform the data (see transforming data) if they are not linear.
2. Calculate the correlation coefficient, which tells you whether the data fit a straight line relationship (and how close the fit is, in statistical terms).
3. If the correlation coefficient is significant, and other conditions are met, proceed to regression analysis, which gives the equation for the line of best fit, then draw this line on your graph.

Transformation of data

1. Proportions and percentages: convert to arcsin values

Certain mathematical assumptions underly all the statistical tests on this site. The most important assumption is that the data are normally distributed and are free to vary widely about the mean - there are no imposed limits. Clearly this is not true of percentages, which cannot be less than 0 nor more than 100. If you have data that are close to these limits, then you need to transform the original data before you analyse them.

One simple way of doing this is to convert the percentages to arcsin values and then analyse these arcsin values. The arcsin transformation moves very low or very high values towards the centre, giving them more theoretical freedom to vary.

[You convert percentages (x) to arcsin values ( q ), where q is an angle for which sin q is Ö x/100 ]

On a calculator:

to get the arcsin value for a percentage (e.g. 50%), divide this by 100 ( = 0.5), take the square root (= 0.7071), then press "sin-1" to get the arcsin value (= 45). [NB: if your calculator gives the result as 0.785 then this is the angle in radians rather than degrees]

to get the arcsin value for a proportion (e.g. 0.4), take the square root (= 0.6325), then press "sin-1" to get the arcsin value (= 39.23).

convert percentages to arcsin values (and back again) by entering a formula into the spreadsheet - Go for it!

2. Logarithmic transformation

Use this for two purposes:

• When fitting a curve to logarithmic data (exponential growth of cells, etc). Take the logarithm of each "growth" value and plot this against time (real values). You can use either natural logarithms or logs to base 10. The data should now show a straight-line relationship and can be analysed using correlation coefficient and regression.
• In Analysis of Variance, when comparing means that differ widely. The reason for this is that an analysis of variance is based on the assumption that the variance is the same across all the data. But usually this will not be true if some means are very small and others are very large - the individual data points for the large mean could vary widely. [For example, a mean of 500 could be made up from 3 values of 100, 400 and 1000, whereas a mean of 50 could not possibly include such wide variation] This problem is overcome by converting the original data to logarithms, squeezing all the data points closer together. Contrary to expectations, this would show significant differences between small and large means that would not be seen otherwise.

3. Converting Percentages to Probits

Some types of data show a sigmoid (S-shaped) relationship. A classic case is in dosage-response curves, for testing antibiotics, pharmaceuticals, etc. To analyse these relationships the "percentage of patients/cells responding to a treatment" can be converted to a "probit" value, and the dosage is converted to a logarithm. This procedure converts an S-shaped curve into a straight-line relationship, which can be analysed by correlation coefficient and regression analysis in the normal way. From the straight-line equation, we can calculate the LD50, ED50, and so on.

The method for doing this in "Excel" is shown below.

Converting between percentage, arcsin and probits in ‘Excel’.

The table below shows part of a page from an ‘Excel’ worksheet. Columns are headed A-F and rows are labelled 1-21, so each cell in the table can be identified (e.g. B2 or F11). Representative % values were inserted in cells B2-B21.

You will now see how to convert these % values into probits or arcsin values, and back again. If you do the relevant conversion in your own spreadsheet, you can then use the probit or arcsin values instead of % values for the statistical tests.

In cell C2 of the spreadsheet. a formula was entered to convert Percentage to Probit values.

The formula (without spaces) is: =NORMINV(B2/100,5,1)

This formula is not seen. As soon as we move out of cell C2 it automatically gives the probit value (in C2) for the percentage in cell B2, seen in the "printout" below. Copying and then pasting this formula into every other cell of column C produces a corresponding probit value (e.g. cell C3 contains the probit of the % in cell B3).

Next, a formula was entered in cell D2 to convert Probit to Percentage, and the above procedure was repeated for all cells in column D.

The formula is: =NORMDIST(C2,5,1,TRUE)*100

The formula entered in cell E2 converts Percentage to Arcsin

The formula is: =ASIN(SQRT(A2/100))*180/PI()

The formula in cell F2 converts Arcsin to Percentage

The formula is: =SIN(E2/180*PI())^2*100

 A B C D E F 1 Percent % to Probit Probit to % % to arcsin arcsin to % 2 0.1 1.91 0.1 1.812 0.1 3 0.5 2.424 0.5 4.055 0.5 4 1 2.674 1 5.739 1 5 2 2.946 2 8.13 2 6 3 3.119 3 9.974 3 7 4 3.249 4 11.54 4 8 5 3.355 5 12.92 5 9 6 3.445 6 14.18 6 10 7 3.524 7 15.34 7 11 8 3.595 8 16.43 8 12 9 3.659 9 17.46 9 13 10 3.718 10 18.43 10 14 50 5 50 45 50 15 96 6.751 96 78.46 96 16 97 6.881 97 80.03 97 17 98 7.054 98 81.87 98 18 99.5 7.576 99.5 85.95 99.5 19 99.99 8.719 99.99 89.43 99.99 20 99.999 9.265 99.999 89.82 99.999 21 99.9999 9.768 100 89.94 99.9999
 CONTENTS WHAT TEST DO I NEED? STATISTICAL TESTS: Student's t-test for comparing the means of two samples Paired-samples test. (like a t-test, but used when data can be paired) Analysis of variance for comparing means of three or more samples: Chi-squared test for categories of data Poisson distribution for count dataCorrelation coefficient and regression analysis for line fitting: TRANSFORMATION of data: percentages, logarithms, probits and arcsin values