[IV 212] updating index rst

Author: NathanielF

docs/source/_static/iv_reg.svg

@@ -111,7 +111,7 @@ Interrupted time series analysis is appropriate when you have a time series of o
 
 Instrumental Variable Regression
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-Interrupted time series analysis is appropriate when you have a time series of observations which undergo treatment at a particular point in time. This kind of analysis has no control group and looks for the presence of a change in the outcome measure at or soon after the treatment time.
+Instrumental Variable regression is an appropriate technique when you wish to estimate the treatment effect of some variable on another, but are concerned that the treatment variable is endogenous in the system of interest i.e. correlated with the errors. In this case an "instrument" variable can be used in a regression context to disentangle treatment effect due to the threat of confounding due to endogeneity.
 
 .. image:: _static/iv_reg.svg