Guest Post by Willis Eschenbach

In the comments to my post called “Inside the Acceleration Factory“, we were discussing how good the satellite measurements of sea surface heights might be. A commenter said:

Ionospheric Delay is indeed an issue. For Jason, they estimate it using a dual frequency technique. As with most everything in the world of satellite Sea Level Rise, there is probably some error in their estimate of delay, but its hard to see why any errors don’t ether cancel or resolve over a very large number of measurements to a constant bias in their estimate of sea level — which shouldn’t affect the estimate of Sea Level Rise.

Keep in mind that the satellites are making more than 1000 measurements every second and are moving their “target point” about 8km (I think) laterally every second. A lot of stuff really will average out over time.

I thought I should write about this common misunderstanding.

The underlying math is simple. The uncertainty of the average (also called the “mean”) of a group of numbers is equal to the standard deviation of the numbers (a measure of how spread out the numbers are), divided by the square root of how many numbers there are. In Mathspeak, this is

where sigma (σ) is the standard deviation and N is how many numbers we’re analyzing.

Clearly, as the number of measurements increases, the uncertainty about the average decreases. This is all math that has been well-understood for hundreds of years. And it is on this basis that the commenter is claiming that by repeated measurements we can get very, very good results from the satellites.

With that prologue, let me show the limits of that rock-solid mathematical principle in the real world.

Suppose that I want to measure the length of a credit card.

So I get ten thousand people to use the ruler in the drawing to measure the length of the credit card in millimeters. Almost all of them give a length measurement somewhere between 85 mm and 86 mm.

That would give us a standard deviation of their answers on the order of 0.3 mm. And using the formula above for the uncertainty of the average gives us:

Now … raise your hand if you think that we’ve just accurately measured the length of the credit card to the nearest three thousandths of one millimeter.

Of course not. And the answer would not be improved if we had a million measurements.

Contemplating all of that has given rise to another of my many rules of thumb, which is:

Regardless of the number of measurements, you can’t squeeze more than one additional decimal out of an average of real-world observations.

Following that rule of thumb, if you are measuring say temperatures to the nearest degree, no matter how many measurements you have, your average will be valid to the nearest tenth of a degree … but not to the nearest hundredth of a degree.

As with any rule of thumb, there may be exceptions … but in general, I think that it is true. For example, following my rule of thumb I would say that we could use repeated measurements to get an estimate of the length of the credit card to the nearest tenth of a millimeter … but I don’t think we can measure it to the nearest hundredth of a millimeter no matter how many times we wield the ruler.

Best wishes on a night of scattered showers,

w.

My general request: when you comment please quote the exact words you are referring to, so we can avoid misunderstandings.