Subtracting times - what date format is needed

[smasterson]

Hi folks,

I am constructing an SPSS database and will need to be able to subtract response times to get time interval data. Will I need to use the format "DD/MM/YYYY hh:mm" if I want to subtract e.g. Response Time x (23:55) from Response Time y (00:04). The correct answer is 9 minutes (nothing wrong with my maths as you can see) so I don't want to end up with an answer or 23hrs 1min. My only concern with using "DD/MM/YYYY hh:mm" is the potential for data entry error, but am I right to assume that using this date/time format is the best way to ensure correct subtraction calculations????

[Melissa Ives]

Use the DATEDIFF function to subtract times....

The DATEDIFF function calculates the difference between two date values
and returns an integer (with any fraction component truncated) in the
specified date/time units. The general form of the expression is:

DATEDIFF(datetime2, datetime1, "unit").
where datetime2 and datetime1 are both date or time format variables (or
numeric values that represent valid date/time values), and "unit" is one
of the following string literal values, enclosed in quotes:

Years
Quarters
Months
Weeks
Days
Hours
Minutes <------------I believe this is the one you want.
Seconds

So it would look something like this...

Compute RTinterval=DATEDIFF(RTy, RTx, MINUTES).

Melissa

-----Original Message-----
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of
smasterson
Sent: Monday, October 08, 2007 8:24 AM
To: [hidden email]
Subject: [SPSSX-L] Subtracting times - what date format is needed

Hi folks,

I am constructing an SPSS database and will need to be able to subtract
response times to get time interval data. Will I need to use the format
"DD/MM/YYYY hh:mm" if I want to subtract e.g. Response Time x (23:55)
from Response Time y (00:04). The correct answer is 9 minutes (nothing
wrong with my maths as you can see) so I don't want to end up with an
answer or 23hrs 1min. My only concern with using "DD/MM/YYYY hh:mm" is
the potential for data entry error, but am I right to assume that using
this date/time format is the best way to ensure correct subtraction
calculations????
--
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[Richard Ristow]

At 09:24 AM 10/8/2007, smasterson wrote:

>I am constructing an SPSS database and will need to be able to
>subtract response times to get time interval data. Will I need to use
>the format "DD/MM/YYYY hh:mm" if I want to subtract e.g. Response Time
>x (23:55) from Response Time y (00:04). The correct answer is 9
>minutes (nothing wrong with my maths as you can see) so I don't want
>to end up with an answer or 23hrs 1min. My only concern with using
>"DD/MM/YYYY hh:mm" is the potential for data entry error, but am I
>right to assume that using this date/time format is the best way to
>ensure correct subtraction calculations????

No, I'm afraid you're not. But the actual situation is more favorable
than you'd anticipate.

You should have your data-entry people enter the dates in whatever
format is most convenient and error-free. You'll read those as
*character* values. However (and this is a very strong recommendation),
your program that reads the data should immediately calculate separate
variables that give the values in SPSS date-time representation.

There are SPSS date and date-time formats to read most common date
representations; use them with the NUMBER function, since SPSS dates
are numeric variables. If you don't have a format that SPSS will read,
you can convert it with string commands.

Then, give the new variables any SPSS date or date-time format you
want. The formats affect only the *display* of these variables, not
their underlying values(*). As Melissa Ives wrote, you can then use
DATEDIFF. Or, simply subtract the values; the difference of two SPSS
date or date-time variables are in seconds.

(*) "Internally, all date format variables are stored as the number of
seconds from 0 hours, 0 minutes, and 0 seconds of Oct. 14, 1582." -
SPSS 15 Command Syntax Reference, p. 47.

[smasterson]

Thank you Richard,

As you have no doubt surmised, I am a novice at data analysis on SPSS so am
learning as I go

Kind regards

Siobhan


Siobhan Masterson
Project Manager - OHCAR
Department of Public Health Medicine
HSE - West
Old St. Eunan's Ward
St. Conal's Hospital
Letterkenny
Co. Donegal
Ireland

Office: 00 353 74 9104460 (main office) x4076
Mobile: [phone]




             Richard Ristow
             <wrristow@mindspr
             ing.com>                                                   To
                                       smasterson
             08/10/2007 20:10          <[hidden email]>,
                                       [hidden email]
                                                                        cc
                                       Melissa Ives <[hidden email]>
                                                                   Subject
                                       Re: Subtracting times - what date
                                       format is needed










At 09:24 AM 10/8/2007, smasterson wrote:

>I am constructing an SPSS database and will need to be able to
>subtract response times to get time interval data. Will I need to use
>the format "DD/MM/YYYY hh:mm" if I want to subtract e.g. Response Time
>x (23:55) from Response Time y (00:04). The correct answer is 9
>minutes (nothing wrong with my maths as you can see) so I don't want
>to end up with an answer or 23hrs 1min. My only concern with using
>"DD/MM/YYYY hh:mm" is the potential for data entry error, but am I
>right to assume that using this date/time format is the best way to
>ensure correct subtraction calculations????

No, I'm afraid you're not. But the actual situation is more favorable
than you'd anticipate.

You should have your data-entry people enter the dates in whatever
format is most convenient and error-free. You'll read those as
*character* values. However (and this is a very strong recommendation),
your program that reads the data should immediately calculate separate
variables that give the values in SPSS date-time representation.

There are SPSS date and date-time formats to read most common date
representations; use them with the NUMBER function, since SPSS dates
are numeric variables. If you don't have a format that SPSS will read,
you can convert it with string commands.

Then, give the new variables any SPSS date or date-time format you
want. The formats affect only the *display* of these variables, not
their underlying values(*). As Melissa Ives wrote, you can then use
DATEDIFF. Or, simply subtract the values; the difference of two SPSS
date or date-time variables are in seconds.

(*) "Internally, all date format variables are stored as the number of
seconds from 0 hours, 0 minutes, and 0 seconds of Oct. 14, 1582." -
SPSS 15 Command Syntax Reference, p. 47.




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