The operations that can be used in the TRANSFORMIN= and TRANSFORMOUT= options are shown in Table 15.1. Operations are applied to each value of the series. Each value of the series is replaced by the result of the operation.
In Table 15.1, or x represents the value of the series at a particular time period t before the transformation is applied, represents the value of the result series, and N represents the total number of observations.
The notation indicates that the argument is an optional integer; the default is 1. The notation window is used as the argument for the moving statistics operators, and it indicates that you can specify either a number of periods n (where n is an integer) or a list of n weights in parentheses. The internal maximum value of the number of periods n is clipped at the number of observations in the series. The notation sequence is used as the argument for the sequence operators, and it indicates that you must specify a sequence of numbers. The notation s indicates the length of seasonality, and it is a required argument.
Table 15.1: Transformation Operations
Syntax 
Result 

+ number 
Adds the specified number : 
number 
Subtracts the specified number : 
* number 
Multiplies by the specified number : 
/ number 
Divides by the specified number : 
ABS 
Absolute value: 
ADJUST 
Indicates that the following moving window summation or product operator should be adjusted for window width 
CD_I s 
Classical decomposition irregular component 
CD_S s 
Classical decomposition seasonal component 
CD_SA s 
Classical decomposition seasonally adjusted series 
CD_TC s 
Classical decomposition trendcycle component 
CDA_I s 
Classical decomposition (additive) irregular component 
CDA_S s 
Classical decomposition (additive) seasonal component 
CDA_SA s 
Classical decomposition (additive) seasonally adjusted series 
CEIL 
Smallest integer greater than or equal to x : 
CMOVAVE window 
Centered moving average 
CMOVCSS window 
Centered moving corrected sum of squares 
CMOVGMEAN window 
Centered moving geometric mean 
for window = number of periods, n: 







for window = weight list, w: 



CMOVMAX n 
Centered moving maximum 
CMOVMED n 
Centered moving median 
CMOVMIN n 
Centered moving minimum 
CMOVPROD window 
Centered moving product 
for window = number of periods, n: 



for window = weight list, w: 



CMOVRANGE n 
Centered moving range 
CMOVRANK n 
Centered moving rank 
CMOVSTD window 
Centered moving standard deviation 
CMOVSUM n 
Centered moving sum 
CMOVTVALUE window 
Centered moving t value 
CMOVUSS window 
Centered moving uncorrected sum of squares 
CMOVVAR window 
Centered moving variance 
CUAVE 
Cumulative average 
CUCSS 
Cumulative corrected sum of squares 
CUGMEAN 
Cumulative geometric mean 
CUMAX 
Cumulative maximum 
CUMED 
Cumulative median 
CUMIN 
Cumulative minimum 
CUPROD 
Cumulative product 
CURANK 
Cumulative rank 
CURANGE 
Cumulative range 
CUSTD 
Cumulative standard deviation 
CUSUM 
Cumulative sum 
CUTVALUE 
Cumulative t value 
CUUSS 
Cumulative uncorrected sum of squares 
CUVAR 
Cumulative variance 
DIF 
Span n difference: 
EWMA number 
Exponentially weighted moving average of with 
smoothing weight number, where : 

. 

This operation is also called simple exponential smoothing. 

EXP 
Exponential function: 
FDIF d 
Fractional difference with difference order d where 
FLOOR 
Largest integer less than or equal to x : 
FSUM d 
Fractional summation with summation order d where 
HP_T lambda 
HodrickPrescott Filter trend component where lambda is the nonnegative filter parameter 
HP_C lambda 
HodrickPrescott Filter cycle component where lambda is the nonnegative filter parameter 
ILOGIT 
Inverse logistic function: 
LAG 
Value of the series n periods earlier: 
LEAD 
Value of the series n periods later: 
LOG 
Natural logarithm: 
LOGIT 
Logistic function: 
MAX number 
Maximum of x and number : 
MIN number 
Minimum of x and number : 
> number 
Missing value if , else x 
>= number 
Missing value if , else x 
= number 
Missing value if , else x 
= number 
Missing value if , else x 
< number 
Missing value if , else x 
<= number 
Missing value if , else x 
MOVAVE n 
Backward moving average of n neighboring values: 


MOVAVE window 
Backward weighted moving average of neighboring values: 


MOVCSS window 
Backward moving corrected sum of squares 
MOVGMEAN window 
Backward moving geometric mean 
for window = number of periods, n: 



for window = weight list, w: 



MOVMAX n 
Backward moving maximum 
MOVMED n 
Backward moving median 
MOVMIN n 
Backward moving minimum 
MOVPROD window 
Backward moving product 
for window = number of periods, n: 



for window = weight list, w: 



MOVRANGE n 
Backward moving range 
MOVRANK n 
Backward moving rank 
MOVSTD window 
Backward moving weighted standard deviation: 


MOVSUM n 
Backward moving sum 
MOVTVALUE window 
Backward moving t value 
MOVUSS window 
Backward moving uncorrected sum of squares 
MOVVAR window 
Backward moving variance 
MISSONLY <MEAN> 
Indicates that the following moving time window 
statistic operator should replace only missing values with the 

moving statistic and should leave nonmissing values unchanged. 

If the option MEAN is specified, then missing values are 

replaced by the overall mean of the series. 

NEG 
Changes the sign: 
NOMISS 
Indicates that the following moving time window 
statistic operator should not allow missing values 

PCTDIF n 
Percent difference of the current value and lag n 
PCTSUM n 
Percent summation of the current value and cumulative sum nlag periods 
RATIO n 
Ratio of current value to lag n 
RECIPROCAL 
Reciprocal: 
REVERSE 
Reverses the series: 
SCALE 
Scales the series between and 
SEQADD sequence 
Adds sequence values to series 
SEQDIV sequence 
Divides the series by sequence values 
SEQMINUS sequence 
Subtracts sequence values to series 
SEQMULT sequence 
Multiplies the series by sequence values 
SET ( ) 
Sets all values of to 
SETEMBEDDED ( ) 
Sets embedded values of to 
SETLEFT ( ) 
Sets beginning values of to 
SETMISS number 
Replaces missing values in the series with the number specified 
SETRIGHT ( ) 
Sets ending values of to 
SIGN 
, 0, or 1 as x is < 0, equals 0, or > 0, respectively 
SQRT 
Square root: 
SQUARE 
Square: 
SUM 
Cumulative sum: 
SUM n 
Cumulative sum of multiples of nperiod lags: 


TRIM n 
Sets to missing a value if or 
TRIMLEFT n 
Sets to missing a value if 
TRIMRIGHT n 
Sets to missing a value if 
Some operators compute statistics for a set of values within a moving time window; these are called moving time window operators. There are centered and backward versions of these operators.
The centered moving time window operators are CMOVAVE, CMOVCSS, CMOVGMEAN, CMOVMAX, CMOVMED, CMOVMIN, CMOVPROD, CMOVRANGE, CMOVRANK, CMOVSTD, CMOVSUM, CMOVTVALUE, CMOVUSS, and CMOVVAR. These operators compute statistics of the values for observations
The backward moving time window operators are MOVAVE, MOVCSS, MOVGMEAN, MOVMAX, MOVMED, MOVMIN, MOVPROD, MOVRANGE, MOVRANK, MOVSTD, MOVSUM, MOVTVALUE, MOVUSS, and MOVVAR. These operators compute statistics of the values .
All the moving time window operators accept an argument specifying the number of periods to include in the time window. For example, the following statement computes a fiveperiod backward moving average of X.
convert x=y / transformout=( movave 5 );
In this example, the resulting transformation is
The following statement computes a fiveperiod centered moving average of X.
convert x=y / transformout=( cmovave 5 );
In this example, the resulting transformation is
If the window with a centered moving time window operator is not an odd number, one more lead value than lag value is included in the time window. For example, the result of the CMOVAVE 4 operator is
You can compute a forward moving time window operation by combining a backward moving time window operator with the REVERSE operator. For example, the following statement computes a fiveperiod forward moving average of X.
convert x=y / transformout=( reverse movave 5 reverse );
In this example, the resulting transformation is
Some of the moving time window operators enable you to specify a list of weight values to compute weighted statistics. These are CMOVAVE, CMOVCSS, CMOVGMEAN, CMOVPROD, CMOVSTD, CMOVTVALUE, CMOVUSS, CMOVVAR, MOVAVE, MOVCSS, MOVGMEAN, MOVPROD, MOVSTD, MOVTVALUE, MOVUSS, and MOVVAR.
To specify a weighted moving time window operator, enter the weight values in parentheses after the operator name. The window width is equal to the number of weights that you specify; do not specify .
For example, the following statement computes a weighted fiveperiod centered moving average of X.
convert x=y / transformout=( cmovave( .1 .2 .4 .2 .1 ) );
In this example, the resulting transformation is
The weight values must be greater than zero. If the weights do not sum to 1, the weights specified are divided by their sum to produce the weights used to compute the statistic.
A complete time window is not available at the beginning of the series. For the centered operators a complete window is also not available at the end of the series. The computation of the moving time window operators is adjusted for these boundary conditions as follows.
For backward moving window operators, the width of the time window is shortened at the beginning of the series. For example, the results of the MOVSUM 3 operator are
For centered moving window operators, the width of the time window is shortened at the beginning and the end of the series due to unavailable observations. For example, the results of the CMOVSUM 5 operator are
For weighted moving time window operators, the weights for the unavailable or unused observations are ignored and the remaining weights renormalized to sum to 1.
Some operators compute cumulative statistics for a set of current and previous values of the series. The cumulative statistics operators are CUAVE, CUCSS, CUMAX, CUMED, CUMIN, CURANGE, CUSTD, CUSUM, CUUSS, and CUVAR.
By default, the cumulative statistics operators compute the statistics from all previous values of the series, so that is based on the set of values . For example, the following statement computes as the cumulative sum of nonmissing values for .
convert x=y / transformout=( cusum );
You can specify a lag increment argument n for the cumulative statistics operators. In this case, the statistic is computed from the current and every previous value. When n is specified these operators compute statistics of the values for .
For example, the following statement computes as the cumulative sum of nonmissing values for odd when is odd and for even when is even.
convert x=y / transformout=( cusum 2 );
The results of this example are
You can truncate the length of the result series by using the TRIM, TRIMLEFT, and TRIMRIGHT operators to set values to missing at the beginning or end of the series.
You can use these functions to trim the results of moving time window operators so that the result series contains only values computed from a full width time window. For example, the following statements compute a centered fiveperiod moving average of X, and they set to missing values at the ends of the series that are averages of fewer than five values.
convert x=y / transformout=( cmovave 5 trim 2 );
Normally, the moving time window and cumulative statistics operators ignore missing values and compute their results for the nonmissing values. When preceded by the NOMISS operator, these functions produce a missing result if any value within the time window is missing.
The NOMISS operator does not perform any calculations, but serves to modify the operation of the moving time window operator that follows it. The NOMISS operator has no effect unless it is followed by a moving time window operator.
For example, the following statement computes a fiveperiod moving average of the variable X but produces a missing value when any of the five values are missing.
convert x=y / transformout=( nomiss movave 5 );
The following statement computes the cumulative sum of the variable X but produces a missing value for all periods after the first missing X value.
convert x=y / transformout=( nomiss cusum );
Similar to the NOMISS operator, the MISSONLY operator does not perform any calculations (unless followed by the MEAN option), but it serves to modify the operation of the moving time window operator that follows it. When preceded by the MISSONLY operator, these moving time window operators replace any missing values with the moving statistic and leave nonmissing values unchanged.
For example, the following statement replaces any missing values of the variable X with an exponentially weighted moving average of the past values of X and leaves nonmissing values unchanged. The missing values are interpolated using the specified exponentially weighted moving average. (This is also called simple exponential smoothing.)
convert x=y / transformout=( missonly ewma 0.3 );
The following statement replaces any missing values of the variable X with the overall mean of X.
convert x=y / transformout=( missonly mean );
You can use the SETMISS operator to replace missing values with a specified number. For example, the following statement replaces any missing values of the variable X with the number 8.77.
convert x=y / transformout=( setmiss 8.77 );
If is a seasonal time series with s observations per season, classical decomposition methods "break down" the time series into four components: trend, cycle, seasonal, and irregular components. The trend and cycle components are often combined to form the trendcycle component. There are two basic forms of classical decomposition: multiplicative and additive, which are show below.
where
is the trendcycle component
is the seasonal component or seasonal factors that are periodic with period s and with mean one (multiplicative) or zero (additive)
is the irregular or random component that is assumed to have mean one (multiplicative) or zero (additive)
For multiplicative decomposition, all of the values should be positive.
The CD_TC operator computes the trendcycle component for both the multiplicative and additive models. When is odd, this operator computes an period centered moving average as follows:
For example, in the case where s=5, the CD_TC s operator
convert x=tc / transformout=( cd_tc 5 );
is equivalent to the following CMOVAVE operator:
convert x=tc / transformout=( cmovave 5 trim 2 );
When is even, the CD_TC s operator computes the average of two adjacent period centered moving averages as follows:
For example, in the case where s=12, the CD_TC s operator
convert x=tc / transformout=( cd_tc 12 );
is equivalent to the following CMOVAVE operator:
convert x=tc / transformout=(cmovave 12 movave 2 trim 6);
The CD_S and CDA_S operators compute the seasonal components for the multiplicative and additive models, respectively. First, the trendcycle component is computed as shown previously. Second, the seasonalirregular component is computed by for the multiplicative model and by for the additive model. The seasonal component is obtained by averaging the seasonalirregular component for each season.
where and . The seasonal components are normalized to sum to one (multiplicative) or zero (additive).
The CD_I and CDA_I operators compute the irregular component for the multiplicative and additive models respectively. First, the seasonal component is computed as shown previously. Next, the irregular component is determined from the seasonalirregular and seasonal components as appropriate.
The CD_SA and CDA_SA operators compute the seasonally adjusted time series for the multiplicative and additive models, respectively. After decomposition, the original time series can be seasonally adjusted as appropriate.
The following statements compute all the multiplicative classical decomposition components for the variable X for s=12.
convert x=tc / transformout=( cd_tc 12 ); convert x=s / transformout=( cd_s 12 ); convert x=i / transformout=( cd_i 12 ); convert x=sa / transformout=( cd_sa 12 );
The following statements compute all the additive classical decomposition components for the variable X for s=4.
convert x=tc / transformout=( cd_tc 4 ); convert x=s / transformout=( cda_s 4 ); convert x=i / transformout=( cda_i 4 ); convert x=sa / transformout=( cda_sa 4 );
The X12 and X11 procedures provides other methods for seasonal decomposition. See Chapter 37: The X12 Procedure, and Chapter 36: The X11 Procedure.
For fractional operators, the parameter, d, represents the order of fractional differencing. Fractional summation is the inverse operation of fractional differencing.
Suppose that X is a fractionally integrated time series variable of order d=0.25. Fractionally differencing X forms a time series variable Y which is not integrated.
convert x=y / transformout=(fdif 0.25);
Suppose that Z is a nonintegrated time series variable. Fractionally summing Z forms a time series W which is fractionally integrated of order .
convert z=w / transformout=(fsum 0.25);
For the rank operators, the ranks are computed based on the current value with respect to the cumulative, centered, or moving window values. If the current value is missing, the transformed current value is set to missing. If the NOMISS option was previously specified and if any missing values are present in the moving window, the transformed current value is set to missing. Otherwise, redundant values from the moving window are removed and the rank of the current value is computed among the unique values of the moving window.
The trades of a particular security are recorded for each weekday in a variable named PRICE. Given the historical daily trades, the ranking of the price of this security for each trading day, considering its entire past history, can be computed as follows:
convert price=history / transformout=( curank );
The ranking of the price of this security for each trading day considering the previous week’s history can be computed as follows:
convert price=lastweek / transformout=( movrank 5 );
The ranking of the price of this security for each trading day considering the previous two week’s history can be computed as follows:
convert price=twoweek / transformout=( movrank 10 );
For the product and geometric mean operators, the current transformed value is computed based on the (weighted) product of the cumulative, centered, or moving window values. If missing values are present in the moving window and the NOMISS operator is previously specified, the current transformed value is set to missing. Otherwise, the current transformed value is set to the product of the nonmissing values within the moving window. If a geometric mean operator is specified for a window of size n, the root of the product is taken. In cases where weights are specified explicitly, both the product and geometric mean operators normalize these exponents so that they sum to one.
The interest rates for a savings account are recorded for each month in the data set variable RATES. The cumulative interest rate for each month considering the entire account past history can be computed as follows:
convert rates=history / transformout=( + 1 cuprod  1);
The interest rate for each quarter considering the previous quarter’s history can be computed as follows:
convert rates=lastqtr / transformout=( + 1 movprod 3  1);
The average interest rate for the previous quarter’s history can be computed as follows:
convert rates=lastqtr / transformout=( + 1 movprod (1 1 1)  1);
For the sequence operators, the sequence values are used to compute the transformed values from the original values in a sequential fashion. You can add to or subtract from the original series or you can multiply or divide by the sequence values. The first sequence value is applied to the first observation of the series, the second sequence value is applied to the second observation of the series, and so on until the end of the sequence is reached. At this point, the first sequence value is applied to the next observation of the series and the second sequence value on the next observation and so on.
Let be the sequence values and let , , be the original time series. The transformed series, , is computed as follows:
where .
The multiplicative seasonal indices are 0.9, 1.2. 0.8, and 1.1 for the four quarters. Let SEASADJ be a quarterly time series variable that has been seasonally adjusted in a multiplicative fashion. To restore the seasonality to SEASADJ use the following transformation:
convert seasadj=seasonal / transformout=(seqmult (0.9 1.2 0.8 1.1));
The additive seasonal indices are 4.4, 1.1, 2.1, and 1.2 for the four quarters. Let SEASADJ be a quarterly time series variable that has been seasonally adjusted in additive fashion. To restore the seasonality to SEASADJ use the following transformation:
convert seasadj=seasonal / transformout=(seqadd (4.4 1.1 2.1 1.2));
For the set operators, the first parameter, , represents the value to be replaced and the second parameter, , represents the replacement value. The replacement can be localized to the beginning, middle, or end of the series.
Suppose that a store opened recently and that the sales history is stored in a database that does not recognize missing values. Even though demand may have existed prior to the stores opening, this database assigns the value of zero. Modeling the sales history may be problematic because the sales history is mostly zero. To compensate for this deficiency, the leading zero values should be set to missing with the remaining zero values unchanged (representing no demand).
convert sales=demand / transformout=(setleft (0 .));
Likewise, suppose a store is closed recently. The demand might still be present; hence, a recorded value of zero does not accurately reflect actual demand.
convert sales=demand / transformout=(setright (0 .));
For the scale operator, the first parameter, , represents the value associated with the minimum value () and the second parameter, , represents the value associated with the maximum value () of the original series (). The scale operator rescales the original data to be between the parameters and as follows:
Suppose that two new product sales histories are stored in variables and and you wish to determine their adoption rates. In order to compare their adoption histories the variables must be scaled for comparison.
convert x=w / transformout=(scale 0 1); convert y=z / transformout=(scale 0 1);
For the moving summation and product window operators, the window widths at the beginning and end of the series are smaller than those in the middle of the series. Likewise, if there are embedded missing values, the window width is smaller than specified. When preceded by the ADJUST operator, the moving summation (MOVSUM CMOVSUM) and moving product operators (MOVPROD CMOVPROD) are adjusted by the window width.
For example, suppose the variable has 10 values and the moving summation operator of width 3 is applied to to create the variable with window width adjustment and the variable without adjustment.
convert x=y / transformout=(adjust movsum 3); convert x=z / transformout=(movsum 3);
The above transformations result in the following relationship between and : , , for because the first two window widths are smaller than 3.
For example, suppose the variable has 10 values and the moving multiplicative operator of width 3 is applied to to create the variable with window width adjustment and the variable without adjustment.
convert x=y / transformout=(adjust movprod 3); convert x=z / transformout=(movprod 3);
The above transformation result in the following: , , for because the first two window widths are smaller than 3.
The moving tvalue operators (CUTVALUE, MOVTVALUE, CMOVTVALUE) compute the tvalue of the cumulative series or moving window. They can be viewed as combinations of the moving average (CUAVE, MOVAVE, CMOVAVE) and the moving standard deviation (CUSTD, MOVSTD, CMOVSTD), respectively.
The percentage operators compute the percent summation and the percent difference of the current value and the . The percent summation operator (PCTSUM) computes . If any of the values of the preceding equation are missing or the cumulative summation is zero, the result is set to missing. The percent difference operator (PCTDIF) computes . If any of the values of the preceding equation are missing or the lag value is zero, the result is set to missing.
For example, suppose variable contains the series. The percent summation of lag 4 is applied to to create the variable . The percent difference of lag 4 is applied to to create the variable .
convert x=y / transformout=(pctsum 4); convert x=z / transformout=(pctdif 4);
The ratio operator computes the ratio of the current value and the value. The ratio operator (RATIO) computes . If any of the values of the preceding equation are missing or the lag value is zero, the result is set to missing.
For example, suppose variable contains the series. The ratio of the current value and the lag 4 value of assigned to the variable . The percent ratio of the current value and lag 4 value of is assigned to the variable .
convert x=y / transformout=(ratio 4); convert x=z / transformout=(ratio 4 * 100);