Step-by-step techniques in SPSS
Whitney I. Mattson 09/15/2010
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How to look at proportions of a behavior
How to look at proportion of co-occurrence
How to look at simple patterns of transition
Using a rate per minute measure
SPSS syntax for the functions described
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Contains
 Repeated rows for each
subject
 Each row corresponds to
the same unit of time
 Multiple variables from a
1 to 5 scale
▪ Missing values represent
no occurrence
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These methods are
 Most applicable to files
in a similar format
 Tools here can be
adapted to other cases
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The more traditional
way:
 Split your file by a break
variable, here id
SORT CASES BY id.
SPLIT FILE LAYERED BY id.
 Run Frequencies
FREQUENCIES VARIABLES=AU1
/ORDER=ANALYSIS.
 This works well
▪ But is limited in what it can tell
us
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An aggregation
approach:
 In Data > Aggregate …
▪ Set your break variable (the
same as the split file)
▪ Create two summaries of each
variable
▪ Weighted N
▪ Weighted Missing Values
▪ Create a new dataset with
only the aggregated variables
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The new file contains
 A row for each subject
 The numerator and denominator
for our proportion
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The proportion can be
calculated with a compute
statement
More time consuming
 Needed for more complex
proportion scores
 Proportions can be analyzed
DATASET DECLARE Agg.
AGGREGATE
/OUTFILE='Agg'
/BREAK=id
/AU1_n=N(AU1)
/AU1_nmiss=NMISS(AU1).
COMPUTE AU1_prop=AU1_n / (AU1_n
+ AU1_nmiss).
EXECUTE.
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Back to the base file
 Compute a value when variables
co-occur
▪ Here when there is one valid case
of variable AU1 and variable AU4
 Aggregate again
▪ Add in summaries of the new
variable
▪ Weighted N
▪ Weighted Missing Values
 Compute the proportion of time
these two variables co-occur
IF (NVALID(AU1)>0 & NVALID(AU4)>0) AU1_AU4=1.
EXECUTE.
DATASET DECLARE Agg.
AGGREGATE
/OUTFILE='Agg'
/BREAK=id
/AU1_n=N(AU1)
/AU1_nmiss=NMISS(AU1)
/AU4_n=N(AU4)
/AU4_nmiss=NMISS(AU4)
/AU1_AU4_n=N(AU1_AU4)
/AU1_AU4_nmiss=NMISS(AU1_AU4).
COMPUTE AU1_AU4_prop=AU1_AU4_n / (AU1_AU4_n +
AU1_AU4_nmiss).
EXECUTE.
We now have a proportion
of the session that AU1
and AU4 co-occur
 Using these same
functions with different
denominators yields other
proportions
 For example
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 If you instead computed
AU1 and AU4 co-occurrence
over AU4 cases
 Proportion of time during
AU4 when AU1 co-occurred
COMPUTE AU1_AU4_during_AU4_prop=AU1_AU4_n /
(AU4_n).
EXECUTE.
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Proportions are helpful in looking at
characteristics of behavior broadly
However, we miss the evolution of sequence
and co-occurrence throughout time
Time-series or lag analysis can tell us how
often certain behaviors transition to certain
other behaviors.
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Using the lag function to get values in
previous rows
lag ( variable name )
 Returns the last row’s value for the specified
variable
 Can be used in compute statements to
compare changes in variables
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Here we use a lag
function to assess a
transition
 When AU11 moves to
AU11 & AU14
▪ This gives us the
frequency that AU14
occurs when AU11 is
already there
IF (NVALID(AU11)>0 & NVALID(lag(AU11))>0 &
NVALID(lag(AU14))<1 & NVALID(AU14)>0)
AU11_to_AU11_AU14=1.
EXECUTE.
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In addition to obtaining a straight frequency
you can also use this transition variable to
 Assess a proportion of a specific transition out of
all transitions
 Summarize several of these variables into a
composite variable of transitions
 Plug these variables into more complex equations
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Here are a few other useful time series variables
you can create:
(All of these are accessible through the Transform > Create Time Series… menu)
 Lead – Returns the value of the variable in the next
row
 Difference – Returns the change in value from the
previous row to the current row
▪ Useful for finding changes in levels within a variable
 In this menu you can easily change how many steps
back or forward (order) your function takes
▪ For example the value two rows previous
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Creating a rate per minute measure can
 Help tell you how often a behavior occurs
▪ While controlling for variation in session duration
 Can be used to summarize changes during
meaningful epochs of time
▪ For example, when Stimulus A is presented, do subjects
increase their onset of Behavior X
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Calculating a rate per
minute
 Create a transition (lag)
variable for behavior
onset
 Use Aggregation to
create:
▪ Frequency of onset
variable
▪ A duration of session
variable
IF (NVALID(AU1)>0 & NVALID(lag(AU1))<1)
AU1_onset=1.
EXECUTE.
DATASET DECLARE Agg.
AGGREGATE
/OUTFILE='Agg'
/BREAK=id
/AU11_onset_n=N(AU1_onset)
/frame_n=N(frame).
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The new aggregated
dataset allows
 Calculation of a rate
per minute variable
(30 for the number of frames per second,
60 for the number of seconds in a minute)
 Comparison across
subjects in rate per
minute
COMPUTE AU11_RPM=AU11_onset_n / (frame_n / (30*60)).
EXECUTE.
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You can also use this same method for
different epochs of time
 Just add more break variables
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For example, I create variable Stim_1 that
signifies when I present a stimuli
I then aggregate by ID and this new
variable…
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Like so…
IF (frame < 500 & frame > 599) Stim_1=1.
EXECUTE.
AGGREGATE
/OUTFILE='Agg'
/BREAK=id Stim_1
/AU1_onset_n=N(AU1_onset)
/frame_n=N(frame).
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We now have a rate
per minute for both
conditions
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Based on the aggregated datasets presented
here you can
 Analyze group differences in
▪ Proportions of behavior
▪ Proportions of behavior co-occurrence
▪ Number of transitions
▪ Rate per minute across meaningful periods of time
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Based on these variable creation techniques
you can
 Combine methods to produce variables which
assess more complex questions
 For example:
▪ Is the proportion of Variable A during Variable B higher
after Event X?
▪ Is the rate of transition per minute from Variable A to
Variable B more frequent when Variable C co-occurs?
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As with any set of analyses, ensure that the
particular variable you are calculating in a
meaningful construct
Thank you for your interest!