paneldatanotes

paneldatanotes - Regression with Panel Data(SW Ch 8 A panel...

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Unformatted text preview: Regression with Panel Data (SW Ch. 8) A panel dataset contains observations on multiple entities (individuals), where each entity is observed at two or more points in time. Examples : • Data on 420 California school districts in 1999 and again in 2000, for 840 observations total. • Data on 50 U.S. states, each state is observed in 3 years, for a total of 150 observations. • Data on 1000 individuals, in four different months, for 4000 observations total. Notation for panel data A double subscript distinguishes entities (states) and time periods (years) i = entity (state), n = number of entities, so i = 1,…, n t = time period (year), T = number of time periods so t =1,…, T Data: Suppose we have 1 regressor. The data are: ( X it , Y it ), i = 1,…, n , t = 1,…, T Panel data notation, ctd. Panel data with k regressors: ( X 1 it , X 2 it ,…, X kit , Y it ), i = 1,…, n , t = 1,…, T n = number of entities (states) T = number of time periods (years) Some jargon… • Another term for panel data is longitudinal data • balanced panel : no missing observations • unbalanced panel : some entities (states) are not observed for some time periods (years) Why are panel data useful? With panel data we can control for factors that: • Vary across entities (states) but do not vary over time • Could cause omitted variable bias if they are omitted • are unobserved or unmeasured – and therefore cannot be included in the regression using multiple regression Here’s the key idea: If an omitted variable does not change over time, then any changes in Y over time cannot be caused by the omitted variable. 8-1 Example of a panel data set: Traffic deaths and alcohol taxes Observational unit: a year in a U.S. state • 48 U.S. states, so n = of entities = 48 • 7 years (1982,…, 1988), so T = # of time periods = 7 • Balanced panel, so total # observations = 7 48 = 336 Variables: • Traffic fatality rate (# traffic deaths in that state in that year, per 10,000 state residents) • Tax on a case of beer • Other (legal driving age, drunk driving laws, etc.) Traffic death data for 1982 Higher alcohol taxes, more traffic deaths? Traffic death data for 1988 Higher alcohol taxes, more traffic deaths? 8-2 Why might there be higher more traffic deaths in states that have higher alcohol taxes? Other factors that determine traffic fatality rate: • Quality (age) of automobiles • Quality of roads • “Culture” around drinking and driving • Density of cars on the road These omitted factors could cause omitted variable bias. Example #1: traffic density. Suppose: (i) High traffic density means more traffic deaths (ii) (Western) states with lower traffic density have lower alcohol taxes • Then the two conditions for omitted variable bias are satisfied. Specifically, “high taxes” could reflect “high traffic density” (so the OLS coefficient would be biased positively – high taxes, more deaths) • Panel data lets us eliminate omitted variable bias when the omitted variables are constant over time within a given state.Panel data lets us eliminate omitted variable bias when the omitted variables are constant over time within a given state....
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paneldatanotes - Regression with Panel Data(SW Ch 8 A panel...

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