How doI calculate the lenght frequency of a fish?

Answer 1

This is an interesting problem in sampling. Sometimes, very short questions need a long answer. I assume you want to know how fish length can vary and you will use a relative frequency plot to evaluate this variation. I also assume that you are going to catch many fish, and measure their lengths. If you catch only a few, your estimates are not going to be very good. Now, fish sizes will vary greatly. Let us suppose that you catch 20 fish. Let us suppose that the fish range from 3" to 15", so you choose a 1" interval, so your histogram has 12 intervals. Let's suppose that you can measure fish length's to the nearest quarter inch. The first interval will be from 3" to less than 4", the next will be from 4" to less than 5", etc. If in your sample of 20 fish, ther are five fish in the first interval, you can say that based on your sample, 25% of the fish have lengths between 3 and 4". There may be some problems, because live small fish are very difficult to measure. A small fish needs to be thrown back in the water. The solution- take a picture of the fish as it lies next to a ruler. If you plan to cook and eat the fish, you can put it in ice and measure it's length later. The histogram will give you a good idea on the size variation of the fish that you catch which might be all that you want. But if you want to know the size variation of the fish in the sea (including the fish that you did not catch), this might be a more difficult problem. I would imagine that underwater cameras can be used for this purpose.