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post hoc tests for nonparametric data

Sep 10, 2010 7:13 AM
(3136 views)

Hi

I have a sinking feeling there is no quick fix to this but does anybody know how to run post hoc tests for nonparametric data in JMP? I am trying to compare 3 groups for differences in mean values such as visual acuity and neither the data nor the residuals are normally distributed in every instance. I have been performing Kruskal-Wallis analyses and then excluding groups to make the comparisons between 2 groups at a time. The problem is that this introduces type I error and, if the data were normally distributed, I would do a post hoc test to control for this.

What options are there to do this in JMP - if any?

Pulling my hair out with frustration at present.

Thanks for your time.

Neil

I have a sinking feeling there is no quick fix to this but does anybody know how to run post hoc tests for nonparametric data in JMP? I am trying to compare 3 groups for differences in mean values such as visual acuity and neither the data nor the residuals are normally distributed in every instance. I have been performing Kruskal-Wallis analyses and then excluding groups to make the comparisons between 2 groups at a time. The problem is that this introduces type I error and, if the data were normally distributed, I would do a post hoc test to control for this.

What options are there to do this in JMP - if any?

Pulling my hair out with frustration at present.

Thanks for your time.

Neil

3 REPLIES 3

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Re: post hoc tests for nonparametric data

Hi Neil - this looks as if it'll need some scripting, I'm afraid. I've just had a go at it myself, and come up with the one below, which should produce the same Kruskal-Wallis P values as the Fit Y by X platform, for any nominal factor X and any number of Y variables. I've tried it out on the Big Class.JMP sample data set, using either Age or Sex as the X factor, and Height and/or Weight as the Y variable(s), and it seems to work, though I've no doubt it could be tidied up a lot. This program doesn't adjust the P values from the pairwise comparisons for multiple testing: the best I can suggest there would be to use the Bonferroni correction on them.

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// Start of program;

DataFile = PickFile("Select Raw Data Table for Kruskal-Wallis Test:",, {"JMP Files|JMP"});

dt = open( DataFile );

dt << minimize window;

VarList = ColumnDialog(Title("Assign Roles:"),

Xfac = ColList("Factor ID", Min Col(1), Max Col(1)),

Yvar = ColList("Response(s)", Min Col(1), DataType(Numeric)),

);

Column(dt, Char(VarList["Xfac"][1])) << data type("Character");

Column(dt, Char(VarList["Xfac"][1])) << set modeling type("Nominal");

xCol = Column(dt, VarList["Xfac"]);

Response_List = VarList["Yvar"];

yCols = {};

for(i=1, i<=nItems(VarList["Yvar"]), i++,

insert into(yCols, Column(dt, Response_List[i]))

);

summarize(xLev=by(xCol)); // We'll need a list of the factor levels later, so create it now;

dt << select all rows;

dts = dt << subset(visible);

dto = Oneway(

Y( eval list( yCols ) ), X( eval list( xCol ) ),

Wilcoxon Test( 1 ), Box Plots( 0 ), Mean Diamonds( 0 ), invisible

);

dtoreport = dto << report;

// DTOREPORT will only be a list if there are at least two response variables;

if(islist(dtoreport),

chisqd = dtoreport[1][NumberColBox(5)] << MakeCombinedDataTable,

chisqd = dtoreport[NumberColBox(5)] << MakeCombinedDataTable

);

dtoreport << close window;

chisqd << add multiple columns("With", 1, Before First, Character);

chisqd << add multiple columns("Compare", 1, Before First, Character);

for(i=1, i<=nrow(chisqd), i++,

column(chisqd, "Compare")[i] = "All"; column(chisqd, "With")[i] = "All"

);

/*

We'll get a different set of summary statistics depending on whether xLev has more

than two levels or not, because if xLev > 2 then it's a chi-squared test, whereas

if there are only two levels the 2-sample test returns a Z statistic.

*/

if(nItems(xLev) > 2,

chisqd << delete columns({"ChiSquare", "DF"});

column(chisqd, "Prob>ChiSq") << set name("P Value");

,

chisqd << delete columns({"s", "Z"});

column(chisqd, "Prob>|Z|") << set name("P Value");

);

chisqd << select all rows;

Significances = chisqd << subset(visible);

close(chisqd, nosave);

Significances << set name("Significances");

column(Significances, "P Value") << format("Fixed Dec", 20, 4);

close(dts, nosave);

/*

Now run every pairwise comparison of all the levels of Xfac,

and append each of them in turn to Significances;

*/

if(nItems(xLev) > 2,

xName = Column(dt, VarList["Xfac"]) << get name;

for(k=1, k<=nItems(xLev)-1, k++,

for(l=k+1, l<=nItems(xLev), l++,

dt << select all rows;

TextToParse = "dt << select where((:" || xName || "==\!"" || xLev[k]

|| "\!")|(:" || xName || "==\!"" || xLev[l] || "\!"));";

// show(TextToParse);

eval(parse(TextToParse));

dts = dt << subset(invisible);

dts << set name("DTS");

xCol = Column(dts, VarList["Xfac"]);

Response_List = VarList["Yvar"];

yCols = {};

for(i=1, i<=nItems(VarList["Yvar"]), i++,

insert into(yCols, Column(dt, Response_List[i]))

);

dto = dts << Oneway(

Y( eval list( yCols ) ), X( eval( xCol ) ),

Wilcoxon Test( 1 ), Box Plots( 0 ), Mean Diamonds( 0 ), invisible

);

dtoreport = dto << report;

// DTOREPORT will only be a list if there are at least two response variables;

if(islist(dtoreport),

chisqd = dtoreport[1][NumberColBox(5)] << MakeCombinedDataTable,

chisqd = dtoreport[NumberColBox(5)] << MakeCombinedDataTable

);

chisqdi = chisqd << subset(invisible);

close(chisqd, nosave);

chisqdi << delete columns({"s", "Z"});

column(chisqdi, "Prob>|Z|") << set name("P Value");

column(chisqdi, "P Value") << format("Fixed Dec", 20, 4);

close(dts, nosave);

chisqdi << add multiple columns("With", 1, Before First, Character);

chisqdi << add multiple columns("Compare", 1, Before First, Character);

for(i=1, i<=nrow(chisqdi), i++,

column(chisqdi, "Compare")[i] = xLev[k]; column(chisqdi, "With")[i] = xLev[l]

);

Significances << Concatenate( chisqdi, Append to First Table );

close(chisqdi, nosave);

)

)

);

// Add a column of significance codes for P <= 0.05 = "*", P <= 0.01 = "**", P <= 0.001="***";

Significances << new column("Sig", Character(10));

for(i=1, i<=nrow(Significances), i++,

column(Significances, "Sig")[i] = if(column(Significances, "P Value")[i] <= 0.001, "***",

if(column(Significances, "P Value")[i] <= 0.01, "**",

if(column(Significances, "P Value")[i] <= 0.05, "*", "")

)

)

);

close(dt, nosave);

// End of program;

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Re: post hoc tests for nonparametric data

Another thought: an alternative way it *might* be possible to tackle it would be to try to find a multiple comparison procedure calculated directly from the rank sums within the overall comparison, as opposed to essentially constructing a new set of pairwise tests from scratch, which is what I've done above. I don't actually have a copy of the book any more, so do check me out on this first if you consider ordering a copy, but I *think* "Nonparametric Statistical Methods" by Myles Hollander & Douglas A. Wolfe (http://www.amazon.com/Nonparametric-Statistical-Methods-Myles-Hollander/dp/product-description/0471190454 ) includes a number of post-hoc pairwise tests for several nonparametric procedures including the Kruskal-Wallis test - and if so, perhaps one of those could be scripted up instead.

Regards, David

Regards, David

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Re: post hoc tests for nonparametric data

JMP 9, coming out next month, will include nonparametric multiple comparisons. These methods are, of course, nonparametric tests, and they control for the Type I rate.

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