<p>There can be several reasons.</p>
<ol>
<li><p>In case the subsamples are not random, the variable may be significant on a subsample and not on another. This would be a case of omitted variables.</p>
</li>
<li><p>The larger a sample, the stronger the statistical strength of the estimate, because the standard error of the estimator goes down with sample size. So it is quite possible that a non-zero coefficient is statistically significant if the sample is large, but the same coefficient is not statistically significant if the sample size is small.</p>
</li>
</ol>

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2021-09-05T15:54:23.713Z
2021-09-05T15:54:23.713Z
Giskard
<p>When I did my research with DiD, I saw that when I split the sample to subsample, <strong>while the coefficient in the whole sample is significant but this is no longer in the subsample</strong>. Can I ask what is the reason behind then?</p>

<econometrics>
Why the significance level of variables of interest decrease in subsampling?
Phil Nguyen
# Why the significance level of variables of interest decrease in subsampling?

(Asked by: username_0 on 2021-09-05)

When I did my research with DiD, I saw that when I split the sample to subsample, **while the coefficient in the whole sample is significant but this is no longer in the subsample**. Can I ask what is the reason behind then?

**Tags:** econometrics

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## 1 answers

### Answer by username_1 on 2021-09-05. Score: 2 | Accepted answer

There can be several reasons.

1. In case the subsamples are not random, the variable may be significant on a subsample and not on another. This would be a case of omitted variables.
2. The larger a sample, the stronger the statistical strength of the estimate, because the standard error of the estimator goes down with sample size. So it is quite possible that a non-zero coefficient is statistically significant if the sample is large, but the same coefficient is not statistically significant if the sample size is small.


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