how do I work out the mean from these figures?

Add together all the values and then divide by the number of values.

by 'divide by number of values' you mean number of bars?

The number times their height

I would have said that it would have been sum of the frequency values that those bars represent, rather than just the number of bars.

 

From what I can see from the charts, you have a range of Beak depths for the 3 places. The hight of the bars will tell you how many beaks are of that depth in each place. You want to get the average beak depth - so you add together the depths of all the beaks and divide by the total number of beaks in the study. OK? Have I understood that properly?

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PS - or as Mokele just said - the number x their height.

can you give me an example? I'm feeling so thick today lol

 

so 2.5 x 0.08 (first bar) + 2nd + 3rd etc = total / total frequency (0.98)

Edited February 26, 200916 yr by dg2008

Quick question here: What are the units on the frequency here? Are the 0.2 and 0.4 ment to represent 0.2 and 0.4 thousand beaks or something?

here is the other information I am given:

 

Figure 2 was produced by Peter Grant, the expert on Darwin’s finches, to illustrate his book on the ecology and evolution of the finches. He and others measured the beak (bill) depth of a large number of specimens of three species of ground finch from three different islands. Beak depth is grouped into categories of 0.25 mm, so that, for example, all birds with beak depths between 3 and 3.25 mm fall into the same category. The y-axis of the figure shows the frequency with which each beak depth category occurs in the population. For example on the island of Los Hermanos, the frequency of a beak depth of 3.5 to 3.75 mm is 0.42. This means that 42% of the sample measured had a beak depth within that category. If the total number of birds examined at each location is large enough it will give a mean that is close to the mean that would be obtained if every single finch in the population was measured. Since the sample in this case was a large one, it is likely to be representative of the whole population. The mean beak depth for each species on each island is indicated by an arrowed vertical line.

 

and having just read the last bit again I take it I just need to read of the line

An example: The last graph, "Los Hermanos".

 

Using the horizontal midpoint of each vertical bar as the beak depth for that bar, I read (beak depth, frequency) pairs as (2.875,0.08), (3.125,0.18), (3.375,0.08), (3.625,0.42), (4.125,0.24).

 

Note well: 0.08+0.18+0.08+0.42+0.24 = 1. Make sure that is the case (otherwise you need to divide by the total frequency).

 

The frequency-weighted mean is 2.875*0.08+3.125*0.18+3.375*0.08+3.625*0.42+4.125*0.24=3.575.