When
Do Monarchs Normally Arrive?
Calcluating Mean Arrival Dates
People always wonder, when will the migration reach me? You can use historic migration records collected in a single place to calculate the "mean" arrival date.
The mean date is the average of all the dates. It's the most common measure of center. To determine the mean, find the sum of the values in the set, then divide by the number of values.
To calculate the mean using calendar dates, it helps to rank them, or put the dates in order, and to assign each date a number. The example on the chart below uses the first arrival dates for Baton Rouge, Louisiana, where Monarchs arrived on 3/3/00, 3/31/99, 3/19/98, 3/26/97.
- Put the arrival dates in order. (This has been done for you.)
- Enter dates on the chart. Separate them like a calendar. Use the order number beside each date for its value.
- Add all the values for all the dates
- Then divide by the number of arrival dates. The result is the order number. Find the order number on the chart and calculate the date. This is the mean arrival date.
- The chart has space for you to calculate the mean arrival date for your home town.
| Order |
Date
of First Sighting |
Date
of First Sighting |
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3/3/00 |
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3/19/98 |
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3/26/97 |
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3/31/99 |
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