Leslie E. Papke and Jeffrey M. Wooldridge, "Econometric Methods for
Fractional Response Variables with an Application to 401(k) Plan
Participation Rates", Journal of Applied Econometrics, Vol. 11, No. 6, 
1996, pp. 619-632.

These data are from the 1987 IRS Form 5500 reports of pension plans with
more than 100 participants.  Data for each plan year are maintained by the
Pension and Welfare Benefits Administration of the U.S. Department of
Labor. There are 4735 observations.

The following variables are printed in columns; each row is a different plan.

1.  prate:  plan participation rate
2.  mrate:  plan match rate
3.  totemp: total firm employees
4.  age:  age of plan, years
5.  sole:  =1 if sole retirement plan

The data are in the file 401kjae.raw, which is zipped in pw-data.zip. 
Note that this file is in DOS format.