Why are my soil infiltration rate averages diferent if I use diferent units?

[Camilo Vallejos]

Hello,

 

I have a table with soil water infiltration rates in minutes per cm and then converted to cm per hour (60/number of minutes). When I calculate the average of both columns, I get diferent measurements.

 

For example:

In min /cm: 300, 6.768, 1.712 average: 102.82

In cm / hour: 0.2, 8.86, 35.04 average: 14.70

 

But 102.82 min/cm does not equal 14.70 cm/hour.

Ive spend hours looking for my mystake but can´t find it, please help.

 

I don´t know if this kind of question is apropiate here. If not please point me to an apropiate forum.

[Strange]

The average of a set of numbers is not equal to the reciprocal of the average of the reciprocals.

Because the reciprocal of the sum of reciprocals is the not the same as the sum.

For example, [math]\frac{1}{\frac{1}{a}+\frac{1}{b}} = \frac{ab}{a+b} \neq a+b[/math]

[Camilo Vallejos]

Thanks a lot. I had to search the definition of reciprocal wich is what I am creating when doing the conversion, now it makes sense.

If I understand correctly, I should use the average of converted units instead of converting the average of the unconverted set. On to check your webpage.