How to Convert String Variables into Numeric Ones?

Introduction & Practice Data File

Converting an SPSS string variable into a numeric one is simple. However, there's a huge pitfall that few people are aware of: string values that can't be converted into numbers result in system missing values without SPSS throwing any error or warning.

This can mess up your data without you being aware of it. Don't believe me? I'll demonstrate the problem -and the solution- on convert-strings.sav, part of which is shown below.

SPSS String To Numeric Data View

SPSS Strings to Numeric - Wrong Way

First off, you can convert a string into a numeric variable in variable view as shown below.

SPSS Convert String To Numeric Variable In Variable View

Now, I never use this method myself because

So What's the Problem?

Well, let's do it rather than read about it. We'll

  1. set empty cells as user missing values for s3;
  2. convert s3 to numeric in variable view;
  3. run descriptives on the result.
*Set empty string as user missing value for s3.

missing values s3 ('').

*Inspect frequency table for s3.

frequencies s3.

*Now manually convert s3 to numeric under variable view.

*Inspect result.

descriptives s3.

*N = 444 instead of 459. That is, 15 values failed to convert and we've no clue why.


SPSS Frequency Table Before Conversion

Note that some values in our string variable have been flagged with “a”. We probably want these to be converted into numbers. We have 459 valid values (non empty cells).

SPSS String To Numeric Descriptives Conversion Errors

After converting our variable to numeric, we ran some descriptives. Note that we only have N = 444. Apparently, 15 values failed to convert -probably not what we want. And we usually won't notice this problem because we don't get any warning or error.

Conversion Failures - Simplest Solution

Right, so how can we perform the conversion safely? Well, we just

In our first example, the frequency table already suggested we must remove the “a” from all values before converting the variable. We'll do just that in a minute.
Although safe, I still think this method is too much work, especially for multiple variables. Let's speed things up by using some handy syntax.

SPSS - String to Numeric with Syntax

The fastest way to convert string variables into numeric ones is with the ALTER TYPE command.This requires SPSS version 16 or over. For SPSS 15 or below, use the NUMBER function. It allows us to convert many variables with a single line of syntax.
The syntax below converts all string variables in one go. We then check a descriptives table. If we don't have any system missing values, we're done.


*Close data without saving and reopen before proceeding.

*Convert all variables in one go.

alter type s1 to s3 (f1) s4 (f6.3).

*Inspect descriptives.

descriptives s1 to s4.

Note: using alter type s1 to s4 (f1). will also work but the decimal places for s4 won't be visible. This is why we set the correct f format: f6.3 means 6 characters including the decimal separator and 3 decimal places as in 12.345. Which is the format of our string values.


SPSS String To Numeric System Missings Descriptives Table

Since we've 480 cases in our data, we're done for s1. However, the other 3 variables contain system missings so we need to find out why. Since we can't undo the operation, let's close our data without saving and reopen it.

Solution 2: Copy String Variables Before Conversion

Things now become a bit more technical. However, readers who struggle their way through will learn a very efficient solution that works for many other situations too. We'll basically

Precisely, we'll flag non empty string values that are system missing after the conversion. As these are at least suspicious, we'll call those conversion failures. This may sound daunting but it's perfectly doable if we use the right combination of commands. Those are mainly STRING, RECODE, DO REPEAT and IF.

Copy and Convert Several String Variables

*Close data without saving and reopen before proceeding.

*Copy all string variables.

string c1 to c4 (a7).

recode s1 to s4 (else = copy) into c1 to c4.

*Convert variables to numeric.

alter type s1 to s3 (f1) s4 (f6.3).

*For each variable, flag conversion failures: cases where converted value is system missing but original value is not empty.

do repeat #conv = s1 to s4 / #ori = c1 to c4 / #flags = flag1 to flag4.
if(sysmis(#conv) and #ori <> '') #flags = 1.
end repeat.

*If N > 0, conversion failures occurred for some variable.

descriptives flag1 to flag4.


SPSS Alter Type Conversion Failures

Only flag3 and flag4 contain some conversion failures. We can visually inspect what's the problem by moving these cases to the top of our dataset.

*Visually inspect why values fail to convert.

sort cases by flag3 (d).

*Some values flagged with 'a'.

sort cases by flag4 (d).

*Some values flagged with 'a' through 'e'.


SPSS String To Numeric Conversion Failures Data View

Remove Illegal Characters, Copy and Convert

Some values are flagged with letters “a” through “e”, which is why they fail to convert. We'll now fix the problem. First, we close our data without saving and reopen it. We then rerun our previous syntax but remove these letters before the conversion.


*Close data without saving and reopen before proceeding.

*Copy all stringvars.

string c1 to c4 (a7).

recode s1 to s4 (else = copy) into c1 to c4.

*Remove 'a' from s3.

compute s3 = replace(s3,'a','').

*Remove 'a' through 'e' from s4.

do repeat #char = 'a' 'b' 'c' 'd' 'e'.
compute s4 = replace(s4,#char,'').
end repeat.

*Try and convert variable again.

alter type s1 to s3 (f1) s4 (f6.3).

*Flag conversion failures again.

do repeat #conv = s1 to s4 / #ori = c1 to c4 / #flags = flag1 to flag4.
if(sysmis(#conv) and #ori <> '') #flags = 1.
end repeat.

*Inspect if conversion succeeded.

descriptives flag1 to flag4.

*N = 0 for all flag variables so we're done.

*Delete copied and flag variables.

delete variables c1 to flag4.


SPSS No Statistics Are Computed Because There Are No Valid Cases

All flag variables contain only (system) missings. This means that we no longer have any conversion failures; all variables have been correctly converted. We can now delete all copy and flag variables, save our data and move on.

Thanks for reading!

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