Estimators - Statistics

, and is it always the variance that is used in determining the S.E.?

if you have an estimate of the mean and for this population of size n, you know:

the population variance
have calculated a (unbiased~n-1 denom)sample variance

Don't know. But probably. Question is too general for me.



You may find this useful.
Stop reading when you get to "correction for finite population" label - it's not relevant to A-level, and will only confuse.



Variance uses "n", whether you're dealing with random variables, or with samples.

When you want an unbiased estimate of the variance of a population, given a sample then you use "n-1"

Hope that helps.

Cheers. On a slightly unrelated note.

If you have a 3- dimensional vector given in component form

How would you find the modulus-argument form (r,theta)

I cant see how to work out theta

In three dimensions you need three coordinates. There is more than one "standard" form.

First ask do you need this? What's it for? I don't recall this being covered at A-level.

Its not come up in my alevel but its quite a basic thing in alevels to be asked to convert a 2-D vector into modulus-argument form. So i was just curious about how to extend this idea to3d. The 2d idea is pretty simple so i assumed, naturally, that the same might be true for an i,j,k vector

With 2D, you only need to specify a distance from the origin and an angle relative to the positive x-axis (that being the usual method), and your point is defined.

In 3D you need three coordinates. You can specify a distance from the origin, but you then need two angles (or an angle & a displacment).

Have a look at spherical coordinates and cylindrical coordinates on wiki - those are the two (three actually as there are two flavours of spherical) main ones.

But I wouldn't concern myself with them in any detail until you have the A-level stuff down solid.