IV regression, no observations, within and between group

[Vera Schmidt]

Dear community,

I have tried many things but due to a thought error, I cannot find the solution. I would be very grateful if you could help me.

I want to analyze the following equation:

redistribution=β0+β1⋅intergenerational mobility1 + u

According to Alesina et al. (2018), intergenerational mobility is significantly correlated with redistribution, so they use an experiment to generate exogenous variation for intergenerational mobility and use this as an IV (Instrumental Variable).

I have coded the experiment so that I have the variables for the control group and the treatment group:

• Control group: mobility1 & redistribution
• Treatment group: mobilitytreatment1 & redistributiontreatment

When I stay within one of the groups, I get a regression output:

reg redistribution mobility1

Source | SS df MS Number of obs = 34
-------------+---------------------------------- F(1, 32) = 0.02
Model | .019916327 1 .019916327 Prob > F = 0.8862
Residual | 30.5977307 32 .956179085 R-squared = 0.0007
-------------+---------------------------------- Adj R-squared = -0.0306
Total | 30.6176471 33 .927807487 Root MSE = .97784

------------------------------------------------------------------------------
redistribu~n | Coefficient Std. err. t P>|t| [95% conf. interval]
-------------+----------------------------------------------------------------
mobility1 | .0027527 .019073 0.14 0.886 -.0360977 .041603
_cons | 6.24398 .2207855 28.28 0.000 5.794255 6.693705
------------------------------------------------------------------------------

But when I try to perform an IV regression, I get an error:

ivregress 2sls redistribution ( mobility1 = mobilitytreatment1)
no observations
r(2000);

When I run another regression to check and stay within the control group, I get another regression output:

ivregress 2sls redistribution ( mobility1 = mobility2)

Instrumental variables 2SLS regression Number of obs = 34
Wald chi2(1) = 0.51
Prob > chi2 = 0.4742
R-squared = .
Root MSE = .95729

------------------------------------------------------------------------------
redistribu~n | Coefficient Std. err. z P>|z| [95% conf. interval]
-------------+----------------------------------------------------------------
mobility1 | .0173522 .0242436 0.72 0.474 -.0301644 .0648687
_cons | 6.134054 .2455078 24.99 0.000 5.652868 6.615241
------------------------------------------------------------------------------
Instrumented: mobility1
Instruments: mobility2

Conclusion: Within group it works, between group it does not work.


What am I doing wrong? Many thanks in advance!