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back to the future comparison tool

This tool implements the Back to the Future analysis between two investments over a five year period. Historical data feeds are sourced from the Google finance database, while Google spreadsheets powers the calculation and generates the plot.

The form expects three inputs. Fill in the first two boxes with your choice of stock or fund symbols. Then, choose an end date for the comparison. Usually you'll select the current day, e.g. , but you might want to choose a date before the mortgage meltdown, if you believe that period is atypical. Or you may want to evaluate how well that fund weathered tough seas, and use today's date. Then hit "SUBMIT". Finally, click on the yellow box below to launch a new window with the calculation's results.

After about a 30 second delay, the calculated BTTF chart will appear as a locked Google spreadsheet in this new window, replacing the old plot from a previous run. The jagged curves are total returns from any single investment date to the end date. The light smooth curves are effective, yearly percentage returns from that day to the end date.

If you plan on holding your fund for a shorter period (say under two years) try this complementary "Moving Holding Period" MHP tool.

There are some limitations:

  • Some funds do not license their data to Google. If the chart appears faulty, check with google.com/finance first to see if the requested data is available.
  • The weekly closing price is sometimes missing or reported on varying dates. Missing datapoints are bridged, and the two curves are aligned in time. The end date is moved to the nearest Wednesday to avoid weekend and holiday effects.
  • Dividends, splits, etc are not (yet) available to include in the return calculation. More critical for some investments than others.
  • If the stock or fund hasn't existed for at least five years, the tool will not generate a complete plot.
  • I average the most recent month's stock price to set the baseline for return calculations- this moderates the effect of an aberrant day's gain/loss, and corrects for missing Google data.
  • The plot in the new window may have been generated by the most recent user of this tool- don't use this tool if you are worried that others will free-ride on your brilliant stock selection.
  • This is a SHARED spreadsheet-- only one person at a time can choose a pair of stocks, so its possible someone else might be selecting and plotting different funds while you are waiting for your selection to appear. Wait a minute and try again.
  • Check for mistyped dates and stock symbols- Google is unforgiving (the stock symbols do not have to be in capital letters, but the date must be in the "month/day/year" format like 6/15/2010).
  • This tool is a bit flaky and occasionally misfires (Google spreadsheets are still relatively primitive, so I had to code around many of its current limitations). USE AT YOUR OWN RISK.
After filling in the form and submitting, you must click here to load the calculated chart in a new window.
If the new chart does not appear within a minute, try clicking this box again.
Make sure your browser allows pop-up windows.


Contact Greg Blonder by email here - Modified Genuine Ideas, LLC.