Re: Re: st: propensity score matching question

[Nyasha Tirivayi <ntirivayi@gmail.com>]

Dear Ariel

Thanks so much for your response. The untreated individuals cannot be
temporally aligned to the treated individuals. However I still think
there is no dose response problem. Hence I will go ahead and try your
suggestion and match anyway.

Thank you very much.

Nyasha Tirivayi
Maastricht
Netherlands

On Wed, Aug 8, 2012 at 6:21 PM, Ariel Linden, DrPH
<ariel.linden@gmail.com> wrote:
> Hi Nyasha,
>
> Is there a reason why you could not match the treated individuals in each
> period to controls in those respective periods? In other words, those
> treated in the first period (July 2009 - Dec 2009) would be matched to
> available untreated individuals during that period.
>
> Note that matching is intended to replicate an RCT (by matching on observed
> characteristics and assuming that there is no meaningful residual
> confounding). Thus, you would be matching on pre-intervention
> characteristics only in each cohort (without looking at the outcomes).
>
> If the untreated individuals do not temporally align with the treatment
> periods, you can still match, since what is of primary importance here is
> that the treated and matched controls are balanced on baseline observed
> characteristics. Of course, if "time" is an important covariate to consider,
> you will have to ensure that the controls are matched temporally.
>
> As for your last question about differing observation periods: that could be
> a major problem if you believe there is a dose-response here. You should
> rely on content expertise to answer that question, but you should also test
> that in the data. If there is a dose-response, you will have to ensure that
> you are comparing treated to controls over the same duration. If you
> arbitrarily "standardize" the time-frame under study (such is the case when
> people "annualize" data), you may be introducing measurement error.
>
> I hope this helps
>
> Ariel
>
>
> Date: Tue, 7 Aug 2012 13:27:50 +0200
> From: Nyasha Tirivayi <ntirivayi@gmail.com>
> Subject: Re: st: propensity score matching question
>
> Dear Peter
>
> Thanks for your response. The study is evaluating the impact of food
> aid on adherence to medication. The hypotheses is that food aid
> improves adherence to medication. I have three groups of the treated
> i.e.  first group was on food aid from july 2009 to dec 2009, second
> group from may 2009 to Jan 2010  and third group from december 2009 to
> July 2010. The first group has about 36 people, the second around 65
> and the last one abour over a 100.
>
> The comparison group has over 1000 people. Since I do not have the
> same baseline for everyone, is propensity score matching still ideal
> for my evaluation? How would I account for the varying times in
> program participation by the three treated group?   If not, what other
> method can I use in Stata.
>
> Kindly advise
>
> Regards
>
> N. Tirivayi
> Maastricht
> Netherlands
>
>
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