How to select a random sample without replacement with imposed condition?

Hi,

I'd like to draw a random sample from my population. My population is a vector x of size (N x 1). My sample should satisfy

sum(x(sample)) = 0.7*sum(x);

So I do not care how many observations I draw from the sample, but I just want to make sure that the resulting sample covers 70 percent of total x in terms of value, not in terms of number.

Thanks, Christian

[Walter Roberson]

Is that exactly 0.7 ? If so then it might not be possible to satisfy the condition.

[Christian]

Good point. Let's say at least 0.7, but as close as possible to 0.7. So stop sampling as soon as the sum is equal or greater than 0.7.

Answer 1

[Walter Roberson]

rp = randperm(N);
loc = find( cumsum(x(rp)) >= 0.7 * sum(x), 1, 'first');
samps = x(rp(1:loc));

[Christian]

Thanks. Very neat.