Applying z-tests and t-tests


z-tests and t-tests

The z-test

Generally, we do not know the value of the standard deviation of the  (σ), and we have to estimate it with the standard deviation of the sample population (s). However, if we do know the standard deviation of the population we can use the z-test. The term "Z-test" is often used to refer specifically to the one-sample location test comparing the mean of a set of measurements to a given constant when the sample variance is known

Step 1: formulating a hypothesis

First, a hypothesis is formulated. There are two hypotheses: the null hypothesis and the alternative hypothesis. The null hypothesis means that the independent variable did not have an effect. The hypothesis implies that there is no change or difference. For the null hypothesis the symbol H0 is used. The H refers to hypothesis, and the 0 refers to zero effect. Next, there is the alternative hypothesis (H1) which indicates that there is a change or difference. In the context of an experiment, it indicates that the independent variable (for example a treatment method for depression) did have an effect on the dependent variable (extent of depression). When the null hypothesis for example is that the mean depression score is 30 in the population of depressed people, the alternative hypothesis can be that the mean does not equal 30 (µ ≠ 30). In some cases, the direction of the difference is also specified. For example, if it is expected that the group that received treatment has a higher mean, it applies: H1: µ1 < µ2. It is for example possible to indicate with H1 that the mean is lower than 30 (µ < 30) or higher than 30 (µ > 30). The latter possibility is in this example superfluous, because it is almost impossible that a treatment will cause an increase in the degree of depression. In this case we would say that H1 is a one-tailed, for the estimated value is less than the reference value; in opposition a two-tailed hypothesis is appropriate if the estimated value may be more than or less than the reference value.

The mathematical representations of the null and alternative hypotheses are defined below:

  • H0μ = m0
  • H1μ ≠ m0 (two-tailed)
  • H1μ > m0 (upper-tailed)
  • H1μ < m0 (lower-tailed)

Step 2: criteria to make a decision

To make a founded decision about the (in)correctness of the null hypothesis, certain criteria have to be used. We use the level of significance or the alfa level (α) as criterion. The alfa level is a limit in the normal distribution that distinguishes between scores with a high chance and scores with a low chance of occurring in the sample, if the hypothesis is true. An alfa of 5% (α = 0.05) implies that there is a 5% chance that the result is found by chance. The alfa level is a chance value that is being used to determine highly unlikely sample results, if the null hypothesis is true.

Inferential statistics

The area that is demarcated with 'extreme low probability values if H0 is true' is called the critical region. The critical region consists of extreme sample values that are highly unlikely if the null hypothesis is true. When a found value falls within this critical region, it differs significantly from the mean and the null hypothesis is rejected. For an alfa of 5%, it implies that 5% of the scores falls within this critical area.

Step 3: collecting data and calculations

Data are collected after the hypotheses are formulated. This way, the data can be tested by means of the hypothesis; the researcher can evaluate the data in an objective manner. After collecting the raw data, the statistics are calculated. The researcher calculates for example the sample mean. The, the mean can be compared to the null hypothesis. To do so, the researcher has to compute a z-score that describes the position of the sample mean compared to the mean of the null hypothesis. The z-score for the sample mean is calculated:

\[z =\frac{(x - \mu)}{\sigma}\]

  • z: how many standard deviations below or above the population mean a raw score is
  • x: sample value
  • µ: population mean
  • σ: population standard deviation

The z-score for testing hypotheses is an example of a test statistic.

Step 4: making a decision

A researcher uses the z-score from the previous step to make a decision about the null hypothesis. The first possibility is that the researcher rejects the null hypothesis. This is the case when the statistics falls within the critical region. This means that there is a significant difference between the sample and the null hypothesis. The statistic is found in the tail of the distribution. Referring to the example of treating depression, it means that the researcher has found that the treatment had a significant effect. The second possibility is that the null hypothesis can not be rejected. This is the case when the statistic does not fall within the critical area.

The t-test

As mentioned before: In general, we do not know the value of σ and we have to estimate it with the sample standard deviation (s). When we replace σ by s, we can not use the z-formula, but we have to use the t-test.

\[t =\frac{\bar{x} - \mu}{\frac{s}{\sqrt n}}\]

  • t: The t-statistic (t-test statistic) for a one sample t-test
  • : sample mean
  • µ: population mean
  • s: standard deviation of sample
  • n: size of sample population

Assumptions for the one-sample t-test

There are two assumptions that have to be met in order to conduct a t-test.

  1. First, the scores in the sample have to be independent observations. That means that one score can not influence another score. The chance on a certain outcome for a score can thus not be influenced by another score.

  2. Second, the population, from which the sample is drawn, has to be normally distributed. In practice, violation of this assumption has little influence on the t-statistic, especially when the sample size is high. With quite small samples, it is nevertheless important that the population is normally distributed. When you are insecure whether the distribution of the population is normal, it is best to use a high sample size.

Effect size of the t-test

The effect size can be computed with Cohen’s d: first the difference between the sample and population mean has to be determined. This has to be divided by the standard deviation of the population. Often, the standard deviation of the population is unknown. Hence, an estimated d is constructed by dividing the difference between sample and population mean by the standard deviation of the sample.

The t-test for independent samples

The t-test is used often to test differences between two independent samples. For example, when we compare the achievements between a control group and an experimental group (which received a treatment). We want to examine whether the difference is large enough to assume that the two samples originate from different populations.

When we compare means of two different populations, we test the null hypothesis H0: µ1 - µ2 = 0. This comprises a sampling distribution of all possible difference scores between the population means. In case of two normally distributed populations, the distribution of the difference scores is also normally distributed. The variance of this distribution can be found with the variance sum law: the variance of the sum of the difference of two independent variables equals the sum of there differences:

\[\sigma_{x \pm y}^2 = \sigma_{x}^2 + \sigma_{y}^2\]

So if the variance of set 1 was 2, and the variance of set 2 was 5.6, the variance of the united set would be 2 + 5.6 = 7.6.

Assumptions for the t-test with two independent samples

  1. The observations in each sample are independent.

  2. The populations from which the samples are drawn, are normally distributed. When the researcher assumes that the populations are not normally distributed, it is advised to use large samples.

  3. The two populations have equal variances. We call this homogeneity of variances. Pooling sampling variance is only useful when both populations have the same variance. This assumption is very important, because a correct interpretation depends upon the research findings. You can check if this assumption is met with Levene’s test in SPSS.

Pooled variance

In general, a t-test also assumes that both samples have the same sample size. When the samples do not have an equal sample size, the outcome is biased towards the small sample. To correct for this, a formula is used that combines the variances: the pooled variance. The pooled variance is found by taking the weighted mean of both variances. The sum of squares of both samples is divided by the degrees of freedom. The degrees of freedom are lower for a smaller sample, so that this smaller sample will receive a lower weight. As mentioned before, the variance of a sample (s2) can be obtained by dividing SS by df.

\[s^2 = \frac{SS}{df}\]

  • s2: variance of a sample
  • SS: sum of squares
  • df: degrees of freedom (n-1)

To calculate the pooled variance (s2p), a different formula is used:

\[s_{p}^2 = \frac{SS_1 + SS_2}{df_1 + df_2}\]

The estimated standard error of M1-M2 is found by taking the square root of the pooled variance devided by number of cases

\[s_{M_1-M_2} = \sqrt{\frac{s_{p}^2}{n_1 + n_2}}\]

Effect size

As mentioned before, Cohen’s d can be computed by dividing the difference between the means by the standard deviation of the population. For two independent samples, the difference between the two samples (M1 – M2) is used to estimate the difference in means. The pooled standard deviation (s2p) is used to estimate the standard deviation of the population. The formula to estimate Cohen’s d is thus:

\[Cohen's \: d = \frac{M_1 - M_2}{\sqrt{s_{p}^2}}\]

Paired t-test

A paired t-test is used when there is a matched design or when there are repeated measures. The paired t-test takes into account that participants in two conditions are similar to each other. In this case, there are two different samples, but each individual from the one sample is matched to an individual from the other sample. Individuals are matched basis on variables that are considered to be important for the study. This causes an increase of the power: if the independent variable truly has an effect, it is more likely that this will be found in the study. The lower the error variance, the higher the power of the experiment. A high power results in a lower pooled standard deviation (sp). The lower the pooled standard deviation, the higher the t-value.

The t-statistic for related samples is, with regard to its structure, similar to the other t-statistics. The main difference is that the t-statistic for matched samples is based upon difference scores instead of raw scores (X-values). Because participants before and after the treatment are examined, each participant has a difference score. The difference score is:

\[D = X_1 - X_2\]

In this formula, the X2 refers to the second measurement (often: after the treatment). When D is a negative number, it implies that the extent of occurrence of the variable X has decreased after the treatment. A researcher tries to examine whether there is a difference between two conditions in the population by using difference scores. He wants to know what would happen when each individual in the population would be measured twice (before and after the treatment). The researcher strives to know what the mean of the difference score (µD) in the population is.

The null hypothesis is that the mean of the difference scores is zero (H0 : µD = 0). According to this hypothesis, it is possible that some individuals in the population have positive difference scores. In addition, it is possible that some individuals have negative difference scores. However, the main question is whether the mean of all difference scores equals zero. The alternative hypothesis H1 states that the mean of the difference scores does not equal zero (H1 : µD ≠ 0).

Assumptions for the paired-samples t-test

  1. The scores within each condition are independent.

  2. The difference scores (D) are normally distributed. Violation of this assumption is not a big matter, as long as the sample sizes are large. For a small sample, this assumption has to be met. A large sample size refers to a sample with at least 30 participants.

If one or more assumptions of the t-test for repeated measures are not met, an alternative test can be used. This is the Wilcoxon-test, in which rank scores are used for comparing difference scores.

Effect size

The two most frequently used measures for effect size are Cohen’s d and r2 (proportion of explained variance). Because Cohen’s d assumes population parameters (d = µD – σD) it is useful to estimate d. The estimated d can be computed by dividing the mean of the difference scores by the standard deviation (d = MD/s). A value higher than 0.8 is considered a large effect. The proportion of explained variance van be computed as: r2 = t2 / t2 + df.

Statistics: Magazines for encountering Statistics

Statistics: Magazines for encountering Statistics

Startmagazine: Introduction to Statistics
Stats for students: Simple steps for passing your statistics courses

Stats for students: Simple steps for passing your statistics courses

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Stats of studentsTheory of statistics

  • The first years that you follow statistics, it is often a case of taking knowledge for granted and simply trying to pass the courses. Don't worry if you don't understand everything right away: in later years it will fall into place and you will see the importance of the theory you had to know before.
  • The book you need to study may be difficult to understand at first. Be patient: later in your studies, the effort you put in now will pay off.
  • Be a Gestalt Scientist! In other words, recognize that the whole of statistics is greater than the sum of its parts. It is very easy to get hung up on nit-picking details and fail to see the forest because of the trees
  • Tip: Precise use of language is important in research. Try to reproduce the theory verbatim (ie. learn by heart) where possible. With that, you don't have to understand it yet, you show that you've been working on it, you can't go wrong by using the wrong word and you practice for later reporting of research.
  • Tip: Keep study material, handouts, sheets, and other publications from your teacher for future reference.

Formulas of statistics

  • The direct relationship between data and results consists of mathematical formulas. These follow their own logic, are written in their own language and can therefore be complex to comprehend.
  • If you don't understand the math behind statistics, you don't understand statistics. This does not have to be a problem, because statistics is an applied science from which you can also get excellent results without understanding. None of your teachers will understand all the statistical formulas.
  • Please note: you will have to know and understand a number of formulas, so that you can demonstrate that you know the principle of how statistics work. Which formulas you need to know differs from subject to subject and lecturer to lecturer, but in general these are relatively simple formulas that occur frequently and your lecturer will tell you (often several times) that you should know this formula.
  • Tip: if you want to recognize statistical symbols you can use: Recognizing commonly used statistical symbols
  • Tip: have fun with LaTeX! LaTeX code gives us a simple way to write out mathematical formulas and make them look professional. Play with LaTeX. Wit that, you can include used formulas in your own papers and you learn to understand how a formula is built up – which greatly benefits your understanding and remembering that formula. See also (in Dutch): How to create formulas like a pro on JoHo WorldSupporter?
  • Tip: Are you interested in a career in sciences or programming? Then take your formulas seriously and go through them again after your course.

Practice of statistics

Selecting data

  • Your teacher will regularly use a dataset for lessons during the first years of your studying. It is instructive (and can be a lot of fun) to set up your own research for once with real data that is also used by other researchers.
  • Tip: scientific articles often indicate which datasets have been used for the research. There is a good chance that those datasets are valid. Sometimes there are also studies that determine which datasets are more valid for the topic you want to study than others. Make use of datasets other researchers point out.
  • Tip: Do you want an interesting research result? You can use the same method and question, but use an alternative dataset, and/or alternative variables, and/or alternative location, and/or alternative time span. This allows you to validate or falsify the results of earlier research.
  • Tip: for datasets you can look at Discovering datasets for statistical research

Operationalize

  • For the operationalization, it is usually sufficient to indicate the following three things:
    • What is the concept you want to study?
    • Which variable does that concept represent?
    • Which indicators do you select for those variables?
  • It is smart to argue that a variable is valid, or why you choose that indicator.
  • For example, if you want to know whether someone is currently a father or mother (concept), you can search the variables for how many children the respondent has (variable) and then select on the indicators greater than 0, or is not 0 (indicators). Where possible, use the terms 'concept', 'variable', 'indicator' and 'valid' in your communication. For example, as follows: “The variable [variable name] is a valid measure of the concept [concept name] (if applicable: source). The value [description of the value] is an indicator of [what you want to measure].” (ie.: The variable "Number of children" is a valid measure of the concept of parenthood. A value greater than 0 is an indicator of whether someone is currently a father or mother.)

Running analyses and drawing conclusions

  • The choice of your analyses depends, among other things, on what your research goal is, which methods are often used in the existing literature, and practical issues and limitations.
  • The more you learn, the more independently you can choose research methods that suit your research goal. In the beginning, follow the lecturer – at the end of your studies you will have a toolbox with which you can vary in your research yourself.
  • Try to link up as much as possible with research methods that are used in the existing literature, because otherwise you could be comparing apples with oranges. Deviating can sometimes lead to interesting results, but discuss this with your teacher first.
  • For as long as you need, keep a step-by-step plan at hand on how you can best run your analysis and achieve results. For every analysis you run, there is a step-by-step explanation of how to perform it; if you do not find it in your study literature, it can often be found quickly on the internet.
  • Tip: Practice a lot with statistics, so that you can show results quickly. You cannot learn statistics by just reading about it.
  • Tip: The measurement level of the variables you use (ratio, interval, ordinal, nominal) largely determines the research method you can use. Show your audience that you recognize this.
  • Tip: conclusions from statistical analyses will never be certain, but at the most likely. There is usually a standard formulation for each research method with which you can express the conclusions from that analysis and at the same time indicate that it is not certain. Use that standard wording when communicating about results from your analysis.
  • Tip: see explanation for various analyses: Introduction to statistics
Statistics: Magazines for understanding statistics

Statistics: Magazines for understanding statistics

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Understanding data: distributions, connections and gatherings
Understanding reliability and validity
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Understanding variability, variance and standard deviation
Understanding inferential statistics
Understanding type-I and type-II errors
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Statistics: Magazines for applying statistics

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