Frequency conversion lets you convert a series of data from one workfile frequency to another. For example you might have quarterly data that you want to use together with annual data, requiring you to convert it from quarterly to annual. In EViews this means that you copy a series from one page of a workfile to another page with a different frequency. Although EViews lets you copy from one workfile to another workfile, it often makes sense to perform the copy between pages of the same workfile.
In this example I will be using the following workfile containing US macro-economic data (downloaded using the handy GetMacroData Add-in):
You can download the workfile from here.
As you can see, this workfile has three pages - an Annual page, a Quarterly page and a Monthly page. The annual page contains real GDP data, the quarterly page contains nominal GDP data, and the monthly page contains unemployment data.
The first thing we'll do is convert realGDP to a quarterly frequency. To do this we select the Annual page, right click on the realgdp series, and select copy:
We then select the quarterly page, right click in some of the white space, and select Paste Special:
This brings up the paste special dialog:
There's a few things to talk about here. The first "Pattern" box lets you specify the name that will be used in the destination page. If you want to keep the same name as the original series (realgdp in our case), just leave that box as it is (i.e. with a "*" in it).
Following that you have a choice whether to paste as a "Series (by Value)" or as a "Link". If you choose to paste as a Series, then the copy will be a one-off event. The Quartely page will have a snapshot of the realgdp data at the time the copy was done. If you choose to paste as a Link, the data in the Quarterly page will permanently be linked to the data in the Annual page - that is whenever the data in the Annual page changes, the data in the Quarterly page will also change.
The right hand side of the dialog lets you specify the frequency conversion options. There are two sets of options - "High to low" and "Low to high". If your source page has a higher date frequency than your destination page, you should set the "high to low" options. If your source page has a lower date frequency than your destination page, you should set the "low to high" options.
Since we are moving from annual to quarterly, we are moving from a lower frequency to a higher one, and so should set the "Low to high" options:
A description of these is:
- Constant-match average: This simply repeats the low value for each of the high observations. i.e. each quarter in 2009 will have the same value as the annual value for 2009.
- Constant-match sum: This simply repeats the low value divided by the number of observations in the high page. i.e. each quarter in 2009 will have the annual value for 2009 divided by 4 (since there are 4 quarters in a year).
- Quadratic-match average: This performs a proprietary local quadratic interpolation of the low frequency data to fill in the high observations.
- Quadratic-match sum: This performs the same proprietary local quadratic interpolation, but this time the low frequency data is divided by the number of observations first.
- Linear-match last: This inserts the low observation value into the last period of the high frequency data, then performs linear interpolation on the missing values. i.e. 2009Q4 will be given the annual 2009 value, 2010Q4 will be given the annual 2010 value, then linear interpolation will fill in 2010Q1, 2010Q2, 2010Q3.
- Cubic-match last: This is the same as linear-match last, but rather than linear interpolation, a cubic spline is used for the interpolation.
For our realgdp series, we'll use Linear-match last (which means we're assuming that real GDP grows at a linear rate throughout the year), and we'll choose to paste as a link. Once we hit OK, you can see the series has been copied to the Quarterly page:
And the frequency conversion is complete. If you open the realgdp link on the quarterly page and open the realgdp series on the annual page, and put them next to each other, you can see how the linear-match last frequency conversion worked: