[R] Combining tapply() and cor.test()?

[Sergey Goriatchev]

 Hello, fellow R-users.

Let me describe the setup first. I have a data.frame, a sample of
which is reported below:

           Company.Name      Periods      Returns   MFR.Factor
350         Wartsila Oyj A      1996-07-31      6.82         0.02
351    Custodia Holding AG  1996-07-31      4.15        -0.02
352           Wartsila Oyj       1996-07-31      7.73         0.09
353           GEA Group AG   1996-07-31    10.12         0.04
354            LEGRAND ORD 1996-07-31     -7.46        -0.20
355 Mayr-Melnhof Karton AG 1996-07-31     4.71        -0.05
356            GEVAERT NPV  1996-08-30      NA          NA
357        NOKIA K FMA2.50  1996-08-30     7.65         0.03
358           Altadis S.A.         1996-08-30     7.65         0.55
359       Metrovacesa S.A.     1996-08-30     4.55        -0.17
360               Oce N.V.          1996-08-30    9.43         0.23

The variable "Periods" is a date object, shows the month.
Variables "Returns" and "MFR.Factor" are numeric.
For each month the number of Returns and MFR.Factors varies, sometimes
it is 350, sometimes 320 etc.

What I need is to use cor.test(Returns, MFR.Factor,...) for each
month, and produce a dataframe with columns: "Period", "cor.estimate",
"p.value".

The simplest way would be with tapply() using variable "Period" as a
factor, but tapply() only applies FUN to just one cell.

What is the most painless way to achieve my objective?

Thank you in advance for your help!

Best,
Sergey