Split file by group i.e. group 1 (non clinical) vs group 2 (clinical)

[Mary McCavert]

Hi there,

Might someone please advise how I split my Stata dataset by 1 variable "Group" which is has two levels 1 (non clinical) vs group 2 (clinical) as I would like to be able to compare those two groups?

I think I'm overthinking this - many thanks in advance for your time and expertise.

[Carlo Lazzaro]

Mary:
welcome to this forum.
Do you mean something along the following lines?

. set obs 2
number of observations (_N) was 0, now 2

. g Group= 0 in 1

. replace Group=1 in 2

. label define Group 0 "non clinical" 1 "clinical"

. label val Group Group

. list

     +--------------+
     |        Group |
     |--------------|
  1. | non clinical |
  2. |     clinical |
     +--------------+

.

[Rich Goldstein]

please provide a data example using -dataex- (see the FAQ) and posted as per the FAQ; the issue is that your description is both vague in certain ways and ambiguous in others and the data example will clear all that up making it much easier to respond in a helpful way without guessing

[William Lisowski]

I think you are indeed overthinking this. In the following example, foreign takes the role of your Group.

. sysuse auto, clear
(1978 Automobile Data)

. summarize weight length if foreign==0

    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
      weight |         52    3317.115    695.3637       1800       4840
      length |         52    196.1346    20.04605        147        233

. summarize weight length if foreign==1

    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
      weight |         22    2315.909    433.0035       1760       3420
      length |         22    168.5455    13.68255        142        193

. by foreign, sort: summarize weight length

------------------------------------------------------------------------------------------------
-> foreign = Domestic

    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
      weight |         52    3317.115    695.3637       1800       4840
      length |         52    196.1346    20.04605        147        233

------------------------------------------------------------------------------------------------
-> foreign = Foreign

    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
      weight |         22    2315.909    433.0035       1760       3420
      length |         22    168.5455    13.68255        142        193

[Ankit Bhardwaj]

Dear STATA Experts,
I have two questions:
Q1: I have a dataset with a group variable with 2 categories: Cirrhosis present and Cirrhosis absent. I want to split it into two parts and calculate descriptive statistics separately for each category.
Q2: I have five groups under etiology variable in my data set, and I need to select and analyze groups 1 and 4 without including the other groups. I've tried searching for a "select command or if command" but haven't been successful. Can you help me determine how to select these two specific groups only for analysis?
Please help.

[Hemanshu Kumar]

Though there is an if command in Stata, most often that is not what you need. Most often (including in your use case) you will use if to choose a subset of observations on which to run another Stata command, such as summarize.

See

help if

You could for instance, do something like

sum myvar if etiology == 1 | etiology == 4

[Ankit Bhardwaj]

Though there is an if command in Stata, most often that is not what you need. Most often (including in your use case) you will use if to choose a subset of observations on which to run another Stata command, such as summarize.

See

help if

You could for instance, do something like

sum myvar if etiology == 1 | etiology == 4

Hemanshu Kumar
I have applied this if command it is giving me the results after adding the groups specified. I need separate statistics for rep78=1 and rep78=2

Repair |
record 1978 | Freq. Percent Cum.
------------+-----------------------------------
1 | 2 2.90 2.90
2 | 8 11.59 14.49
3 | 30 43.48 57.97
4 | 18 26.09 84.06
5 | 11 15.94 100.00
------------+-----------------------------------
Total | 69 100.00

. sum price if rep78==1| rep78==2

Variable | Obs Mean Std. dev. Min Max
-------------+---------------------------------------------------------
price | 10 5687 3216.375 3667 14500

When I have selected rep78=1 and rep78=2, it is adding the numbers and giving combined statistics.

[Hemanshu Kumar]

help by

[Harrison Ochieng]

Not much of an expert, but you can consider using `preserve`,`keep if` and `restore`. Might be helpful in analysing them separately. Before anything else keep a copy of your data

[Ankit Bhardwaj]

Not much of an expert, but you can consider using `preserve`,`keep if` and `restore`. Might be helpful in analysing them separately. Before anything else keep a copy of your data

thanks

[Ankit Bhardwaj]

help by

thank you

[Carlo Lazzaro]

Ankit:
you may want to consider what follows:

. use "C:\Program Files\Stata18\ado\base\a\auto.dta"
(1978 automobile data)

*Reply to your 1st question*
. bysort foreign: sum price

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> foreign = Domestic

    Variable |        Obs        Mean    Std. dev.       Min        Max
-------------+---------------------------------------------------------
       price |         52    6072.423    3097.104       3291      15906

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> foreign = Foreign

    Variable |        Obs        Mean    Std. dev.       Min        Max
-------------+---------------------------------------------------------
       price |         22    6384.682    2621.915       3748      12990

*Reply to your 2nd question*
. gen butler=0 if rep78<=3

. replace butler=1 if rep78>3

. bysort butler: sum price

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> butler = 0

    Variable |        Obs        Mean    Std. dev.       Min        Max
-------------+---------------------------------------------------------
       price |         40    6243.675     3425.43       3291      15906

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> butler = 1

    Variable |        Obs        Mean    Std. dev.       Min        Max
-------------+---------------------------------------------------------
       price |         34        6073    2315.435       3748      12990


.

[Ankit Bhardwaj]

Ankit:
you may want to consider what follows:

. use "C:\Program Files\Stata18\ado\base\a\auto.dta"
(1978 automobile data)

*Reply to your 1st question*
. bysort foreign: sum price

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> foreign = Domestic

Variable | Obs Mean Std. dev. Min Max
-------------+---------------------------------------------------------
price | 52 6072.423 3097.104 3291 15906

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> foreign = Foreign

Variable | Obs Mean Std. dev. Min Max
-------------+---------------------------------------------------------
price | 22 6384.682 2621.915 3748 12990

*Reply to your 2nd question*
. gen butler=0 if rep78<=3

. replace butler=1 if rep78>3

. bysort butler: sum price

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> butler = 0

Variable | Obs Mean Std. dev. Min Max
-------------+---------------------------------------------------------
price | 40 6243.675 3425.43 3291 15906

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-> butler = 1

Variable | Obs Mean Std. dev. Min Max
-------------+---------------------------------------------------------
price | 34 6073 2315.435 3748 12990


.

Thank you for your response. I appreciate the answer to my first question. I would like to rephrase my second question. In a hypothetical scenario, if I have three treatment groups (grp 1, grp 2, and grp 3) in a single column, and I want to compare the weight between grp 1 and grp 3 using an independent t-test, without considering grp 2. Do I need to create a separate column for grp 1 and grp 3 in order to perform the analysis, or is there a way to select the desired groups in a tool like SPSS and have the remaining group automatically excluded from the analysis? I would greatly appreciate your insights on this matter.

[Carlo Lazzaro]

Ankit:

 use "C:\Program Files\Stata18\ado\base\a\auto.dta"
(1978 automobile data)

. gen butler=0 if rep78==3


. replace butler=1 if rep78==4

. ttest price, by(butler) unequal

Two-sample t test with unequal variances
------------------------------------------------------------------------------
   Group |     Obs        Mean    Std. err.   Std. dev.   [95% conf. interval]
---------+--------------------------------------------------------------------
       0 |      30    6429.233    643.5995     3525.14    5112.924    7745.542
       1 |      18      6071.5    402.9585    1709.608    5221.332    6921.668
---------+--------------------------------------------------------------------
Combined |      48    6295.083    427.0852    2958.933    5435.899    7154.268
---------+--------------------------------------------------------------------
    diff |            357.7333    759.3392               -1172.108    1887.574
------------------------------------------------------------------------------
    diff = mean(0) - mean(1)                                      t =   0.4711
H0: diff = 0                     Satterthwaite's degrees of freedom =  44.5217

    Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
 Pr(T < t) = 0.6801         Pr(|T| > |t|) = 0.6399          Pr(T > t) = 0.3199

.

[Andrew Musau]

In a hypothetical scenario, if I have three treatment groups (grp 1, grp 2, and grp 3) in a single column, and I want to compare the weight between grp 1 and grp 3 using an independent t-test, without considering grp 2. Do I need to create a separate column for grp 1 and grp 3 in order to perform the analysis, or is there a way to select the desired groups in a tool like SPSS and have the remaining group automatically excluded from the analysis? I would greatly appreciate your insights on this matter.

To add to Carlo's illustration, note that the t-test is a pairwise test, so you cannot have a larger sample than that implied by two groups. In a situation with more than two groups, you need to use the -if- qualifier, as the following example illustrates.

sysuse auto, clear
*1 vs. 2
ttest mpg if inlist(rep78, 1, 2), by(rep78)

*1 vs. 4
ttest mpg if inlist(rep78, 1, 4), by(rep78)

Res.:

. *1 vs. 2

. 
. ttest mpg if inlist(rep78, 1, 2), by(rep78)

Two-sample t test with equal variances
------------------------------------------------------------------------------
   Group |     Obs        Mean    Std. err.   Std. dev.   [95% conf. interval]
---------+--------------------------------------------------------------------
       1 |       2          21           3    4.242641   -17.11861    59.11861
       2 |       8      19.125    1.328768    3.758324    15.98296    22.26704
---------+--------------------------------------------------------------------
Combined |      10        19.5    1.166667    3.689324    16.86082    22.13918
---------+--------------------------------------------------------------------
    diff |               1.875    3.021731               -5.093125    8.843125
------------------------------------------------------------------------------
    diff = mean(1) - mean(2)                                      t =   0.6205
H0: diff = 0                                     Degrees of freedom =        8

    Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
 Pr(T < t) = 0.7239         Pr(|T| > |t|) = 0.5522          Pr(T > t) = 0.2761

. 
. 
. 
. *1 vs. 4

. 
. ttest mpg if inlist(rep78, 1, 4), by(rep78)

Two-sample t test with equal variances
------------------------------------------------------------------------------
   Group |     Obs        Mean    Std. err.   Std. dev.   [95% conf. interval]
---------+--------------------------------------------------------------------
       1 |       2          21           3    4.242641   -17.11861    59.11861
       4 |      18    21.66667     1.16316     4.93487    19.21261    24.12072
---------+--------------------------------------------------------------------
Combined |      20        21.6    1.067215     4.77273    19.36629    23.83371
---------+--------------------------------------------------------------------
    diff |           -.6666667    3.651484               -8.338149    7.004816
------------------------------------------------------------------------------
    diff = mean(1) - mean(4)                                      t =  -0.1826
H0: diff = 0                                     Degrees of freedom =       18

    Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
 Pr(T < t) = 0.4286         Pr(|T| > |t|) = 0.8572          Pr(T > t) = 0.5714