# Re: st: Why include leads (in a model with lags)?

 From David Greenberg To statalist@hsphsun2.harvard.edu Subject Re: st: Why include leads (in a model with lags)? Date Thu, 01 Oct 2009 15:30:06 -0400

```Sometimes leads are included on the basis of the following reasoning: The future cannot cause its past, so if there is a relation between, say x[t+1] and y[t], it is not causal. Consequently it must reflect some type of non-causal relationship such as reverse causality or spuriousness. If that is the case it may raise questions about the nature of a relationship between x[t-1] and y[t].
- David Greenberg, Sociology Department, New York University

----- Original Message -----
From: Misha Spisok <misha.spisok@gmail.com>
Date: Wednesday, September 30, 2009 8:47 pm
Subject: st: Why include leads (in a model with lags)?
To: statalist@hsphsun2.harvard.edu

> Hello, Statalist!
>
> My question in brief is, "What does including leads in a model that
> 'should' only have lags tell me?"
>
> My intuition is that if leads are statistically signficant, then
> something funky is going on (e.g., "How could future crime rates
> 'predict' current law enforcement expenditures?", or "How could future
> unemployment rates 'predict' current GDP growth?").  However,
> "something funky" is not intellectually satisfying.
>
> I am considering a model of the following form:
>
> y_t = b_0 + b_1*X_t + b_2*X_t-1 + b_3*X_t-2 + u_t
>
> It was suggested to me to use lead values of X, e.g., estimating a
> model of the form,
>
> y_t = b_0 + b_1*X_t + b_2*X_t-1 + b_3*X_t-2 + b_4*X_t+1 + b_5*X_t+2 +
> e_t
>
> If the coefficients b_4 or b_5 are statistically different from zero,
> should this be cause for alarm?  If so, why?
>
> Is there a command in Stata (or other references, a tutorial, etc.)
> that can help me understand this (possible) problem better?
>
> Best,
>
> Misha
> Using Stata 10.1
> *
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