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Generating a random allocation sequence in SPSS

In the following, we will show you how to generate a random allocation sequence in

SPSS (for more information about random allocation sequences please see Chapter

13 of the book! "s in the coin tossing e#ample in the book, we shall set up a random

sequence for $% participants being allocated to two conditions! &he first thing you

need to do is open a new SPSS data file! 'our system should look like this

'ou then need to set up a new )ariable! It does not matter what you call it as we will

not actually be using it anyway! *e ha)e called ours +1-


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*e need to let SPSS know how many random numbers we want generated and so

we ha)e to set up the required number of rows in the data file! *e want $% numbers

and so we need to set up $% rows (or +cases-! If you click on the Editmenu and then

the Insert Casesoption


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'ou should notice that a full.stop +!- appears in the first row of the 1 )ariable! *e

ha)e thus set up one case! *e want $% and so we can simply copy and paste this to

the following 1/ rows in the 1 column to get the following


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*e now want to generate the random numbers! 'ou should click on the Transform,

Computemenu items and you will be presented with the following dialog bo#! &ype in

a new )ariable name into the Target Variablebo# (we ha)e named ours

RandomNumber and then go to the bo# labelled Function Groupand scroll down

until you see the Random Numbersoption and click on this

'ou should notice that a range of entries appears in the Functions and Special

Variablesbo# below it! &here are se)eral ways of generating random numbers! *e

want to generate numbers that ha)e equal probabilities of being selected (a bit like

tossing a coin where heads and tails ha)e equal probabilities of occurring and so we

need to choose the RV.Uniformoption and then click on the arrow to the upper left of

this bo#! 'ou will notice that the following is entered into the Numeric Expression bo#

Select theRandomNumbersand

then the0!niformoptions


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RV.UNIFR!"#$#%

2otice the two question marks in the brackets! 'ou ha)e to type two numbers in

place of the question marks! *hat this e#pression will do is create random numbers

which fall between the two numbers that you place in the brackets! *e are going to

generate random numbers between % and 1 and so these are the )alues that we will

replace the questions marks with i!e!

RV.UNIFR!"&$'%

nce you ha)e done this, the dialog bo# should look like this

Click on (and the random numbers will be generated


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'ou should be able to see that SPSS has generated random numbers between % and

1 (to $ decimal places! *e can now use these numbers to generate our random

allocation sequence! *e can say that if the number is below !4 then the participant is

in the Inter)ention condition and if it is abo)e !4 then they are in the control condition!

*e can get SPSS to work this out for us by using the Transform$ Recode into a

different )ariableoption


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5ighlight the RandomNumber)ariable and mo)e it to the Input Variable *+ utput

Variablebo#! &hen type a )ariable name (e!g! Condition into the Namesection of the

utput Variable section and click on the C,angebutton! &he dialog bo# should look

like this

'ou then need to click on the ld and Ne- Valuesbutton! 'ou be presented with a

new dialog bo#


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6irst of all, we are going to tell SPSS that all the )alues between % and %!4 should be

coded as +1-! &o do this, click on the Range$ /EST t,roug, )alueoption and then

type %!4 into the rele)ant bo#! &hen type a +1- into the Valuebo# in the Ne- Value

section of the dialog bo#

'ou should then click on the0ddbutton! 2e#t you want to tell SPSS to label the

)alues from %!4 to 1 as condition +$-! Click on the Range$ )alue t,roug, 1IG1EST


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option and type %!4 into the bo# below it! &ype +$- into the Value boxand then click on

the0ddbutton! &he dialog bo# should now look like this

&his is set up fine now! Click on Continuefollowed by (! 'ou will see that a new

)ariable is set up in your data file and this has the two conditions labelled as 1 or $


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&o make this more ob)ious you can set up the )alue labels in the Condition)ariable

so that 1 7 Inter)ention and $ 7 Control and then click on the icon at the top of the

screen that looks like a road sign! 'our data file will then look like this

'ou can see from this that we would allocate the first two participants to the

inter)ention condition and the ne#t two to the control condition and so on!

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