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Calculating daily trend


When you first start keeping your log, the very first day, enter your weight in the ``Trend'' column as well as the ``Weight'' column. Thereafter, calculate the number for the ``Trend'' column as follows:

  1. Subtract yesterday's trend from today's weight. Write the result with a minus sign if it's negative.
  2. Shift the decimal place in the resulting number one place to the left. Round the number to one decimal place by dropping the second decimal and increasing the first decimal by one if the second decimal place is 5 or greater.
  3. Add this number to yesterday's trend number and enter in today's trend column.

For example, here's a log for November 1990, You're about to make the entry for November 4th. You've just weighed yourself at 171.5, and entered that number in the weight column. Your log now looks like this:

                   November  1990

Date      Day      Weight      Trend       Rung
----      ---      ------      -----       ----
  1       Thu       172.5      173.5         1
  2       Fri       171.5      173.3         1
  3       Sat       172        173.2         1
  4       Sun       171.5      _____         1
  5       ___      ______      _____        ___
  6       ___      ______      _____        ___

To compute the trend number, subtract yesterday's trend number (173.2) from today's weight (171.5):

- 173.2


Next, shift the decimal point in the difference one position to the left, giving -0.17. This number is rounded to one decimal place by looking at its second decimal place and adding one to the first decimal if it's five or more. Since the second decimal place is 7, we round the number to -0.2. Finally, this number is added to yesterday's trend number (173.2). Since we're adding a negative number, the result is less:

+ -0.2


This result, 173.0, is entered as the trend number for the 4th.

When you begin a new log sheet for the next month, copy the trend number for the last day of the previous month to the line right below the ``Trend'' column heading of the new log. When you compute the trend for the first day of the new month, use that entry as the previous day's trend number.

This may seem a lot of trouble at first, but once you get accustomed to it, you can calculate the new trend number in a few seconds.

(Those conversant with mathematics will recognise this as an ``exponentially smoothed moving average with 10% smoothing,'' and the instructions above as the wordy equivalent of the expression Tn=Tn-1+0.1(Wn-Tn-1) where Tn is the trend number for day n and Wn is the weight number for day n. If you're using a calculator to compute the trend, use this formula. Complete mathematical definitions of various forms of moving averages are given on page [Ref]. The trick of avoiding division by choosing a smoothing percentage of 10% and shifting the decimal place instead has been used for decades by financial analysts making stock and commodity price charts.)

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By John Walker