1. What is a Time Series and why are they Important?

Length of time series

How long a time series do we need? It depends on what we are trying to observe/understand, and what we are going to use the time series for. Some applications may only require a very short set of measurements, but if we only use a short series, we might miss some of the picture.

Sea level again

The plots on this page show the tidal elevations for Dover. If we only take two days worth of data (right), we can see that the tidal cycle is clearly represented. High tide values are about the same, although there is a decrease in high tide over the two days.

The second plot on the right shows a weeks worth of data - does this give us a better idea of what is going on?

The third plot (below) shows a months worth of data. The pattern of spring and neap tides appears.

If we want to know what the mean high tide level is, which plot will give us the best answer?
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Dover tide gauge
One-month tidal record of the sealevel for Dover.
Source: Proudman Oceanographic Laboratory


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Dover tide gauge
Two-days tidal record of the sealevel for Dover.
Source: Proudman Oceanographic Laboratory
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Dover tide gauge
Seven-days tidal record of the sealevel for Dover.
Source: Proudman Oceanographic Laboratory

How long should a time series be?

Unfortunately the answer is often longer than the one you have, or it depends on what you want to know.