Presentation on theme: "My contact details Colin Gray Room S2 (Thursday mornings, especially)"— Presentation transcript:
1My contact details Colin Gray Room S2 (Thursday mornings, especially) address:Telephone: (27) 2234Don’t hesitate to drop in or me.
2This afternoon’s programme 2.05 – A short talk.3.00 – 3.20 A break for coffee.3.20 – Running an analysis with PASW Statistics 17. (PASW stands for Predictive Analytics Software. This is the new name for SPSS Statistics 17 since IBM’s takeover.)
3SESSION 1 The one-way Analysis of Variance (ANOVA)
4The simplest ANOVA design is known as the COMPLETELY RANDOMISED or ONE-FACTOR, BETWEEN SUBJECTS experiment.
5A one-factor, between subjects experiment There are five GROUPS of participants or SUBJECTS.Each participant is RANDOMLY assigned to ONE and ONLY ONE of several different groups or conditions.This type of experiment is said to be of COMPLETELY RANDOMISED, ONE-FACTOR BETWEEN SUBJECTS or ONE-WAY design.An experiment of this design produces INDEPENDENT SAMPLES of scores.
6The one-way ANOVAThe ANOVA of data from a one-factor BETWEEN SUBJECTS experiment is known as the ONE-WAY ANOVA.The one-way ANOVA must be sharply distinguished from the one-factor WITHIN SUBJECTS (or REPEATED MEASURES) ANOVA, which is appropriate when each participant is tested under every condition.That experimental design produces RELATED SAMPLES of scores.The between subjects and within subjects ANOVAs are predicated upon different statistical interpretations, or MODELS of the data.
7Results of the experiment (the RAW DATA) Each column contains the scores of the ten participants who were tested under one particular condition.
9Populations versus samples We have some data on skilled performance.We are actually interested in the performance of people IN GENERAL under these conditions.The POPULATION is the reference set of all possible observations.Our research question is about a population.Our data are merely a subset or SAMPLE from the population.
10Sampling errorDraw a large number of samples of fixed size from a population and calculate the mean M and standard deviation SD for each sample.The values of M and SD will vary from sample to sample.Sampling implies SAMPLING VARIABILITY.If we take the values of the sample statistics for characteristics of the population, we shall be in error. So sampling variability is usually termed SAMPLING ERROR.
11Parameters versus statistics STATISTICS are characteristics of samples.PARAMETERS are characteristics of populations.We use Greek letters to denote parameters; we use Roman letters to denote statistics.
12The null hypothesisThe null hypothesis states that, in the population, all five means have the same value.In other words, the null hypothesis states that none of the drugs has any effect.It’s as if everyone were performing under the Placebo condition.
13Sampling error?The values of the means in our data set vary considerably, casting doubt upon the null hypothesis.On the other hand, we should expect sampling error to result in differences among the five sample means.Are the obtained differences so large as to amount to evidence against H0?
14Deviation scoresA deviation score is a score from which the mean has been subtracted.Deviation scores have the very important property that they sum to zero.
16Breakdown of the total deviation j is the number of the group to which a participant was assigned: j = 1, 2, 3, 4, 5, corresponding to the Placebo, Drug A, Drug B, Drug C and Drug D groups, respectively.So M1 is the Placebo mean, M2 is the mean for the Drug A group, M3 is the mean for the Drug B group, …, M5 is the mean for the Drug D group.
17The total sum of squares The total sum of squares is the sum of the squares of all the total deviation scores.It is a measure of the TOTAL VARIABILITY of the scores.
18Summing and squaringStarting with the breakdown of the total deviation and summing and squaring over all scores, we have …
19Product terms? Where has the product term gone? It has disappeared because deviations about the mean sum to zero.
20Breakdown (or partition) of the total sum of squares
21Interpretation of the partition The between groups deviation (and sum of squares) partly reflects any differences that there may be among the population means.It also reflects random influences such as sampling error and other contributors to the “noisiness” of the data. These influences are collectively known as ERROR.The within groups deviation and within groups sum of squares reflect only error.
22How the one-way ANOVA works The variance BETWEEN the treatment means is compared with the average variance of scores around their means WITHIN the treatment groups.The comparison is made with a statistic called F.
23Mean squaresIn the ANOVA, a variance estimate is known as a MEAN SQUARE, which is expressed as a SUM OF SQUARES (SS), divided by a parameter known as the DEGREES OF FREEDOM (df).
25Degrees of freedomThe term DEGREES OF FREEDOM is borrowed from physics.The degrees of freedom of a system is the number of constraints necessary to determine its state completely.Deviations of n values about their mean sum to zero. So if you know (n – 1) deviations, you know the nth deviation.The sum of squares of the n deviations has only (n – 1) degrees of freedom.
26What the F statistic is measuring The building-block for MSbetween is the between groups deviation of the group mean from the grand mean.The building-block for MSwithin is the within groups deviation of a score from its group mean.
27Repeated sampling Suppose the null hypothesis is true. Imagine the experiment were to be repeated thousands and thousands of times, with fresh samples of participants each time.There would be thousands and thousands of data sets, from each of which a value of F could be calculated.The values of F would vary considerably. The distribution of F with repeated sampling is known as its SAMPLING DISTRIBUTION.
28Expected valueThe expected value of a statistic such as F is its long run mean value with repeated sampling.The expected value of F, written as E(F), is thus the mean of the sampling distribution of the statistic.If the null hypothesis is true and there are no differences among the population means, the expected value of F is close to (though not exactly) unity.
34Degrees of freedom of the between groups mean square SSbetween is calculated from only 5 different values.But deviations about the mean sum to zero, so if you know FOUR deviations about the grand mean, you know the fifth.The between groups mean square has FOUR degrees of freedom.
36The value of FThe value of F is clearly much greater than the expected value of close to 1 under the null hypothesis.
37Range of variation of FThe F statistic is the ratio of two sample variances.A variance can take only non-negative values.So the lower limit for F is zero.There is, however, no upper limit for F.
38Specifying the sampling distribution To test the null hypothesis, you must be able to locate the value of F you obtained from your data in its theoretical sampling distributionTo specify the correct distribution of F (or any other test statistic), you must assign values to properties known as PARAMETERS.
39Parameters of FRecall that the t distribution has ONE parameter: the DEGREES OF FREEDOM (df ).The F distribution has TWO parameters: the degrees of freedom of the between groups and within groups mean squares, which we shall denote by dfbetween and dfwithin, respectively.
40The correct F distribution We shall specify an F distribution with the notation F (dfbetween, dfwithin).We have seen that in our example, dfbetween = 4 and dfwithin = 45.The correct F distribution for our test of the null hypothesis is therefore F(4, 45).This specifies a whole POPULATION of values of F with which we can compare our own value.
43The critical regionIf the value of F is greater than the 95th percentile, reject the null hypothesis.
44The p-value of FThe p-value is the probability of a value at least as extreme as the one obtained – the grey area.If the p-value is less than .05, you’re in the critical region and the null hypothesis is rejected.
45The ANOVA summary table The results of the ANOVA are displayed in the ANOVA summary table.The table contains:a list of sources of variance.the degrees of freedom of each source.its sum of squares.its mean square.the value of F.the p-value of F.
47Reporting the resultThere is a correct format for reporting the results of a statistical test. This is described in the APA publications manual.Never report a p-value as ‘.000’ – that’s not acceptable in a scientific report.Write,“With the alpha-level set at .05, F is significant: F(4, 45) = 9.09; p <.01.”Some prefer to give p-values to three places of decimals.You are now expected to include a measure of EFFECT SIZE as well. More on this later.
48Lisa DeBruine’s guidelines Lisa DeBruine has compiled a very useful document describing the most important of the APA guidelines for the reporting of the results of statistical tests.I strongly recommend this document, which is readily available on the Web.Sometimes the APA manual is unclear. In such cases, Lisa has opted for what seems to be the most reasonable interpretation.If you follow Lisa’s guidelines, your submitted paper won’t draw fire on account of poor presentation of your statistics!
50ANOVA or t test?We can compare the two means by using an independent-samples t test.But what would happen if, instead of making a t test, we were to run an ANOVA to test the null hypothesis of equality of the means?
51Same result! Observe that F = t2 . Observe, however, that the p-value is the same for both tests.The ANOVA and the independent-samples t test are EXACTLY EQUIVALENT and result in the same decision about the null hypothesis.
52Implications of a significant F Our F test has shown a significant effect of the Drug factor. What can we conclude?We can say that the null hypothesis is false.That means that, in the population, the means do not all have the same value.But it does not tell us WHICH differences among our group means are significant.
53Making comparisons among the five treatment means
54Planned comparisonsSuppose that, before you ran your experiment, you had planned to make a set of specified comparisons among the treatment means.For example, you might have planned to compare the mean of the Placebo group with the mean for each of the drug conditions.I am going to describe how such comparisons are made.I am also going to compare the properties of different comparison sets.
55Simple and complex comparisons A comparison between any two of the array of 5 means is known as a SIMPLE comparison.Comparisons between the Placebo mean and each of the others are simple comparisons.But you might want to compare, not single means, but aggregates (means) of means.For example, you might want to compare the Placebo mean with the mean of the four drug means.Such comparisons between aggregates are known as COMPLEX comparisons.
57Non-independent comparisons The simple comparison of M5 with M1 and the complex comparison are not independent or ORTHOGONAL.The value of M5 feeds into the value of the average of the means for the drug groups.
58Systems of comparisons We shall now look at the properties of different sets or systems of comparisons.Which comparisons are independent or ORTHOGONAL?How much VARIANCE can we attribute to different comparisons?How can we test a comparison for SIGNIFICANCE?
59Linear functionsY is a linear function of X if the graph of Y upon X is a straight line.For example, temperature in degrees Fahrenheit is a linear function of temperature in degrees Celsius.
60A linear (straight line) function The coefficient of C is the SLOPE of the line and the constant is the INTERCEPT, which is 320 from the zero point along the vertical (Fahrenheit) axis.
61Equation of a straight line Where b0 and b are the slope and intercept, respectively.
62The general linear equation You have p independent variables, x1, x2, …, xp.The general linear equation is:
63Linear contrastsAny comparison can be expressed as a sum of terms, each of which is a product of a treatment mean and a coefficient such that the coefficients sum to zero.When so expressed, the comparison is a LINEAR CONTRAST, because it has the form of a linear function.It looks artificial at first, but this notation enables us to study the properties of systems of comparisons among the treatment means.
64The complex comparison of the Placebo mean with the mean of the means of the four drug conditions can be expressed as a linear function of the five treatment means …
70Drop the M’s and just work with the coefficients
71Helmert contrastsCompare the first mean with the mean of the other means.Drop the first mean and compare the second mean with the mean of the remaining means. Drop the second mean.Continue until you arrive at a comparison between the last two means.
72Helmert contrasts… Our first contrast is 1, -¼, -¼, -¼, -¼ Our second contrast is0, 1, -⅓ , -⅓, -⅓Our third contrast is0, 0, 1, -½, -½Our fourth (and final) contrast is0, 0, 0, 1, -1
74Orthogonal contrastsThe first Helmert contrast in no way constrains the value of the second, because the first mean has been dropped.The first two contrasts do not affect the third, because the first two means have been dropped.This is a set of four independent or ORTHOGONAL contrasts.
75The orthogonal property As with all contrasts, the coefficients in each row sum to zero.In addition, the sum of the PRODUCTS OF CORRESPONDING COEFFICIENTS in any pair of rows is zero.This means that we have an ORTHOGONAL contrast set.
76Size of an orthogonal set In general, for an array of k means, you can construct a set of, at most, k – 1 orthogonal contrasts.In the present ANOVA example, k = 5, so the rule tells us that there can be no more than 4 orthogonal contrasts in the set.Several different orthogonal sets, however, can often be constructed for the same set of means.
77Contrast sums of squares We have seen that in the one-way ANOVA, the value of SSbetween reflects the sizes of the differences among the treatment means.In the same way, it is possible to measure the importance of a contrast by calculating a sum of squares which reflects the variation attributable to that contrast aloneWe can use an F statistic to test each contrast for significance.
79Formula for a contrast sum of squares In the numerator, the value of the contrast is squared and multiplied by the number of participants in the group.In the denominator, is the sum of the squares of the coefficients in the contrast.
80Here, once again, is our set of Helmert contrasts, to which I have added the values of the five treatment means
86Non-orthogonal contrasts Contrasts don’t have to be independent.For example, you might wish to compare each of the four drug groups with the Placebo group.What you want are SIMPLE CONTRASTS.
87Simple contrasts These are linear contrasts – each row sums to zero. But they are not orthogonal – in any pairing of rows, the sum of products of corresponding coefficients is not zero.The contrast sums of squares will not sum to the between groups sum of squares.
88Testing a contrast sum of squares for significance
89Two approaches A contrast is a comparison between two means. You can therefore make an F test or you can make a t test.The two tests are equivalent.
90Degrees of freedom of a contrast sum of squares A contrast sum of squares compares two means; even though one mean may be an aggregate of several others.A contrast sum of squares, therefore, has ONE degree of freedom, because the two deviations from the grand mean sum to zero.
91Since a contrast sum of squares has one degree of freedom, …
92Degrees of freedom of F The numerator df = 1. The denominator df is just the df for the error term in the full ANOVA (45)
95The equal-n caseWith equal groups, the formula for t simplifies to:
96In the context of ANOVAThis test incorporates the pooled estimate of the population variance, with greater degrees of freedom. The test has more POWER than one based only on the data from two groups.
100Heterogeneity of variance: the Behrens-Fisher statistic With marked heterogeneity of variance, the usual pooling of the sample variances is abandoned and the test statistic is:
101The Behrens-Fisher problem What is the value of the degrees of freedom of T?If n1 > n2, the degrees of freedom of T lies between n2 – 1 and the traditional value of n1 + n2 – 2.Several solutions to the problem of finding the degrees of freedom more precisely have been proposed.
102The Welch-Satterthwaite solution This is the solution used by PASW.Others have been proposed more recently.
103In this case, testing with T doesn’t make much difference
104SummaryA contrast is a comparison between two means, so its sum of squares has ONE degree of freedom.The contrasts can therefore be tested with either F or t. (F = t2.)If the contrasts (Helmert) form an orthogonal set, the contrast sums of squares sum to the value of SSbetween.
107Comparison with SPSS 16From the user’s point of view, PASW Statistics 17 is similar to SPSS 16.Some commands now appear in different menus than they did in SPSS 16.There are some new options.Some old problems and pitfalls remain.The same general user guidelines apply to all versions of PASW and SPSS.
108The Data Editor In SPSS/PASW, there are two display modes: 1. VARIABLE VIEW. This contains information about the variables in your data set.2. DATA VIEW. This spreadsheet-like display contains your numerical data, which are referred to as “values” by PASW: as far as we are concerned, “values” are always numbers. (We shall not be working with string variables such as lists of names or cities.)WORK IN VARIABLE VIEW FIRST, becausethat will make it much easier to enter and view your data in Data View.you can improve the quality of the output.
109Variable names versus variable labels Variable names are for your convenience while working in the Data Editor.They can be very cryptic, so long as you know what they mean.Variable names appear only in Data View.Variable LABELS are transparent and should be understandable by an outsider. They will appear in the output.
110Grouping variables: assign value labels Values are numbers. They might be scores; but they also include the arbitrary code numbers making up GROUPING VARIABLES.Value labels tell you what the code numbers mean.They are ESSENTIAL.If you do not assign value labels to the code numbers and you leave the data file for some time, you won’t remember whether 1 and 2 were male and female, respectively, or vice versa.
111Assigning value labels to values while in Variable View
112Displaying value labels You can see the labels in Data View by clicking on the luggage ticket in the View menu.
113Levels of measurementSPSS/PASW classifies data according to the LEVEL OF MEASUREMENT. There are 3 levels:SCALE data, which are measures on an independent scale with units. Heights, weights, performance scores, counts and IQs are scale data. Each score has ‘stand-alone’ meaning. Two more or less equivalent terms are CONTINUOUS and INTERVAL.ORDINAL data, which are RANKS. A rank has meaning only in relation to the other individuals in the sample. It has no ‘stand-alone’ meaning.NOMINAL data, which are assignments to categories. (So-many males, so-many females.) Nominal data are records of CATEGORICAL or QUALITATIVE variables.
115The drug experiment again Each column contains the scores of the ten participants who were tested under one particular condition.Notice that each line contains data from several subjects/participants. This won’t do for PASW!
116SPSS/PASW data sets One line for each case/participant/subject. The columns are variables.We need a variable for the scores.We need a grouping variable to indicate treatment group or category membership.For the data from the drug experiment, we shall need only two variables (columns): one for the scores, the other for group membership.
119PointsAs always with PASW data files, each row contains data on one person only.Only two columns are needed.One column contains code numbers (values) indicating group membership.The other contains the scores that the participants achieved.
123Beware of the means plot! With some default graphs, SPSS/PASW follows aesthetic guidelines.As a result, some graphs can be highly misleading.Here is the means plot for another data set from an experiment of the same design.
125A false picture!The table of means shows miniscule differences among the five group means.The p-value of F is very high – unity to two places of decimals.Nothing going on here!
126A microscopic vertical scale Only a microscopically small section of the scale is shown on the vertical axis: 10.9 to 11.4!Even small differences among the group means look huge.
127Into the Chart EditorDouble-click on the image to get into the Chart Editor.Double-click on the vertical axis to access the scale specifications.
128Specify zero as the minimum point Uncheck the minimum value box and enter zero as the desired minimum point.Click Apply.
129Zero point now included The effect is dramatic!The profile is now as flat as a pancake.The graph now accurately depicts the results.Always be suspicious of graphs that do not show the ZERO POINT on the VERTICAL SCALE.
130Simple contrasts with SPSS Here are the entries for the first contrast, which is between the Placebo and Drug A groups.Notice that you must enter five coefficients.Below that is the entry for the final contrast between the Placebo and Drug D groups.
131The resultsIn the column headed ‘Value of Contrast’, are the differences between pairs of treatment means.For example, Drug A mean minus Placebo mean = = Drug D – Placebo = – 8.00 = 5.00.
132Save time with Syntax!A computing package can be told to run specified analyses by means of a system of written instructions known as CONTROL LANGUAGE.In PASW/SPSS, the language is known as SYNTAX.If you have to run the same procedure again and again, you should create the appropriate syntax file and save it.At your next session, you need only open the data file and run the whole analysis using the syntax file instead of filling in all the boxes and pressing all the buttons again.
133The Paste buttonIn the ANOVA dialog, simply transfer the group and score variables. That orders the basic analysis.Click the Paste button at the foot of the One-way ANOVA dialog.
138Choose some optionsObserve what happens to the syntax file when we press the appropriate buttons and choose a means plot, descriptives, Tukey multiple-comparisons and order Helmert planned contrasts.
139I press buttons to order Descriptives and a profile plot.
145To enter the whole set …Don’t return to the One-way ANOVA dialog yet. Click the Next button and enter the next row of contrast coefficients.Continue until the whole set of contrasts has been entered, then click the Continue button to return to the One-way ANOVA dialog.Now look at the syntax file.
147Don’t write, just paste! Notice that I haven’t written anything. I have produced all this syntax merely by pressing the Paste button.But I can easily adapt the analysis for other variables and data sets.
148In summaryThe one-way ANOVA tests the null hypothesis of equality of ALL the means by comparing the variance between groups with the variance within groups.Further analysis is necessary for the testing of comparisons among individual group means.Report the results of your analysis in APA format, following Lisa DeBruine’s guidelines.In SPSS/PASW, work in Variable View first.In the ANOVA output, watch out for the profile plots.Save time with syntax!