The Penn World Table (homepage at pwt.econ.upenn.edu) is a rich macroeconomic panel dataset, spanning 152 countries over the years 1950–1992. The data are available in gretl format; please see the gretl data site (this is a free download, although it is not included in the main gretl package).
Example 6-1 below opens pwt56_60_89.gdt, a subset of the pwt containing data on 120 countries, 1960–89, for 20 variables, with no missing observations (the full data set, which is also supplied in the pwt package for gretl, has many missing observations). Total growth of real GDP, 1960–89, is calculated for each country and regressed against the 1960 level of real GDP, to see if there is evidence for "convergence" (i.e. faster growth on the part of countries starting from a low base).
Example 6-1. Use of the Penn World Table
open pwt56_60_89.gdt (* for 1989 (last obs), lag 29 gives 1960, the first obs *) genr gdp60 = RGDPL(-29) (* find total growth of real GDP over 30 years *) genr gdpgro = (RGDPL - gdp60)/gdp60 (* restrict the sample to a 1989 cross-section *) smpl -r YEAR=1989 (* Convergence? Did countries with a lower base grow faster? *) ols gdpgro const gdp60 (* result: No! Try inverse relationship *) genr gdp60inv = 1/gdp60 ols gdpgro const gdp60inv (* No again. Try dropping Africa? *) genr afdum = (CCODE = 1) genr afslope = afdum * gdp60 ols gdpgro const afdum gdp60 afslope