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Re: st: graph results of bitest stratified with by(var)
From
Nick Cox <[email protected]>
To
"[email protected]" <[email protected]>
Subject
Re: st: graph results of bitest stratified with by(var)
Date
Fri, 24 Jan 2014 14:20:45 +0000
<>
This is triply mysterious:
1. You refer to -svret- and -tabout-, but these are user-written
commands (both from SSC), as you are asked to explain.
2. On the face of it, -statsby- is a superior solution for saving
results groupwise, as -svret- saves only the last set of results and
so it use would necessitate reading your data in again.
3. It is not correct that -svret- saves only the first values.
Compare these results, which show that once the loop over groups
is completed, the results saved are those for the last (second) values
calculated, as should be expected: each time around the loop, previous
r-class results are overwritten.
. sysuse auto.dta, clear
(1978 Automobile Data)
. gen group=1 if price<5000
(37 missing values generated)
. replace group=2 if price>=5000
(37 real changes made)
. levelsof group, local(levels)
1 2
. foreach l of local levels {
2. bitest foreign=.2 if group == `l'
3. }
Variable | N Observed k Expected k Assumed p Observed p
-------------+------------------------------------------------------------
foreign | 37 8 7.4 0.20000 0.21622
Pr(k >= 8) = 0.467034 (one-sided test)
Pr(k <= 8) = 0.685908 (one-sided test)
Pr(k <= 6 or k >= 8) = 0.836862 (two-sided test)
Variable | N Observed k Expected k Assumed p Observed p
-------------+------------------------------------------------------------
foreign | 37 14 7.4 0.20000 0.37838
Pr(k >= 14) = 0.009277 (one-sided test)
Pr(k <= 14) = 0.996629 (one-sided test)
Pr(k <= 1 or k >= 14) = 0.011938 (two-sided test)
. svret r, format(%8.2f)
. l
+----------------------------------------------------+
| r_k_opp r_p r_k r_P_p r_N r_p_l r_p_u |
|----------------------------------------------------|
1. | 1 0.01 14 0.20 37 1.00 0.01 |
+----------------------------------------------------+
I can't advise on exact syntax for -tabout-, which I don't use.
Nick
[email protected]
On 24 January 2014 07:09, Michael McCulloch <[email protected]> wrote:
> Thanks Nick & Richard,
> Here's a shot at using -statsby- that gets me what I want.
>
> sysuse auto.dta, clear
> gen group="a" if price<5000
> replace group="b" if price>=5000
> bitest foreign=.2 if group=="a"
> bitest foreign=.2 if group=="b"
> statsby, clear by(group): bitest foreign=.2
> l group N p, clean noobs
>
>
> Then I tried:
>
> sysuse auto.dta, clear
> gen group=1 if price<5000
> replace group=2 if price>=5000
> levelsof group, local(levels)
> foreach l of local levels {
> bitest foreign=.2 if group == `l'
> }
> svret r, format(%8.2f)
> l
>
> I have two questions:
>
> 1. Why in this -svret- attempt does the list return only the first value of group?
> 2. Is it possible to use -tabout- to make the list into a nice table showing both values of group?
>
>
> Best wishes,
> Michael McCulloch
>
> --
> Pine Street Foundation, since 1989
> 124 Pine Street | San Anselmo | California | 94960-2674
> P: (415) 407-1357 | F: (206) 338-2391 | http://www.PineStreetFoundation.org
>
> On Jan 23, 2014, at 10:33 AM, Richard Goldstein wrote:
>
>> true ;-)
>>
>> On 1/23/14, 1:22 PM, Nick Cox wrote:
>>> I see, but you can just calculate that from the -statsby- results.
>>>
>>> Nick
>>> [email protected]
>>>
>>>
>>> On 23 January 2014 18:19, Richard Goldstein <[email protected]> wrote:
>>>> 1. Michael, if you look at -h levelsof- there is an example showing its
>>>> use with -foreach-
>>>>
>>>> 2. Nick, I did not suggest -statsby- because Michael asked for something
>>>> that is not in the return list (r(k)/r(N)) and I don't think that this
>>>> is allowed with -statsby-, but maybe I'm wrong about that?
>>>>
>>>> Rich
>>>>
>>>> On 1/23/14, 1:07 PM, Nick Cox wrote:
>>>>> -search foreach- does exactly what you ask, namely point to sources on
>>>>> -foreach-.
>>>>>
>>>>> But -help statsby- does even better.
>>>>>
>>>>> Here's a dopey example.
>>>>>
>>>>> sysuse auto, clear
>>>>> statsby N=r(N) k=r(k) p_l=r(p_l) p_u=r(p_u) , by(rep78): bitest
>>>>> foreign=.2
>>>>> list
>>>>>
>>>>> Nick
>>>>> [email protected]
>>>>>
>>>>> On 23 January 2014 17:55, Michael McCulloch <[email protected]>
>>>>>
>>>>>> Thank you Richard, that's exactly what I'm wanting to achieve.
>>>>>> I understand now that -bysort- clears the scalars at each re-call.
>>>>>>
>>>>>> Can you point me to primers so I can learn how to wrap this into a -foreach- / -levelsof- loop?
>>>>>
>>>>>
>>>>> On Jan 23, 2014, at 5:45 AM, Richard Goldstein wrote:
>>>>>>
>>>>>>> If I understand what you want correctly, you cannot do it with bysort
>>>>>>> because each time you do the test the set of returned values (the
>>>>>>> "r()"'s) will be replaced and the old ones lost
>>>>>>>
>>>>>>> you can do this within a -foreach- loop (you may need -levelsof- first)
>>>>>>> in which you quietly do the -bitest- and then list your results for that
>>>>>>> test and then do the next -bitest-, etc.
>>>>>>>
>>>>>>> here is an example of how to use the returned values:
>>>>>>>
>>>>>>> . sysuse auto
>>>>>>>
>>>>>>> . bitest foreign=.2
>>>>>>>
>>>>>>> Variable | N Observed k Expected k Assumed p Observed p
>>>>>>> -------------+------------------------------------------------------------
>>>>>>> foreign | 74 22 14.8 0.20000 0.29730
>>>>>>>
>>>>>>> Pr(k >= 22) = 0.029904 (one-sided test)
>>>>>>> Pr(k <= 22) = 0.984075 (one-sided test)
>>>>>>> Pr(k <= 7 or k >= 22) = 0.041800 (two-sided test)
>>>>>>> r; t=0.09 8:39:38
>>>>>>>
>>>>>>> . di r(N) _skip(2) r(P_p) _skip(2) r(k)/r(N) _skip(2) r(p)
>>>>>>> 74 .2 .2972973 .04179963
>>>>>>>
>>>>>>> I have not put headers on the columns and have not done several other
>>>>>>> things you might want (e.g., print format for results) but this should
>>>>>>> give the basic idea, assuming I have correctly understood you
>>>>>
>>>>> On 1/23/14, 12:57 AM, Michael McCulloch wrote:
>>>>>
>>>>>>>> I am using bitest for a two-sided test on whether the mean of varB is different than 0.2, and testing on each level of varA:
>>>>>>>> bysort varA: bitest varB=.2
>>>>>>>>
>>>>>>>> varA has ~30 values. I wish to display these in a table (showing N, observed p,
>>>>>>> expected p, and the two-sided p-value), without manual cut-and-paste, as
>>>>>>> the test
>>>>>>> will be used to monitor an ongoing training program.
>>>>>>>>
>>>>>>>> I note that the results of bitest are stored as:
>>>>>>>> r(N) number N of trials
>>>>>>>> r(P_p) assumed probability p of success
>>>>>>>> r(k) observed number k of successes
>>>>>>>> r(p_l) lower one-sided p-value
>>>>>>>> r(p_u) upper one-sided p-value
>>>>>>>> r(p) two-sided p-value
>>>>>>>>
>>>>>>>> However, I do not know how one uses these r(**) values. Can anyone suggest how one
>>>>>>> would go about this?
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