Create country-year/month/day panels consistent with the COW or Gleditsch & Ward lists of independent states. I mainly use this for merging different data sources:

  1. Create a master template that reflects one of the independent states lists.
  2. For each data source, normalize to a copy of the master template. Doing this by source makes it easier to identify and address issues like missing values or observation for non-independent states.
  3. In the end, merge everything together. Since all the inputs are already normalized to a proper state panel list, there should be no issues.

What the package does:

  1. It contains the Gleditsch and Ward (G&W) as well as the Correlates of War (COW) state system membership lists.


    Search them with sfind, this can be helpful for manual coding:

  2. You can use it to build a country-year template that matches either the COW or G&W state lists.

  3. Visualize missing/non-independent cases in your data. See this related blog post for more details.

    # for the underlying data
    mm <- missing_info(polity, "polity", "ccode", "date", "year", statelist = "COW")


The package is on CRAN and can be installed with:

Or to install from GitHub:



For the Gleditsch and Ward (G&W) state data:

Gleditsch, Kristian S. & Michael D. Ward. 1999. “Interstate System Membership: A Revised List of the Independent States since 1816.” International Interactions 25: 393-413.

For the Correlates of War (COW) state data:

Correlates of War Project. 2017. “State System Membership List, v2016.” Online,