Excluding posts of certain categories

I’ve been trying to exclude posts from certain categories in my blog.

I have found some success using the following code:

<?php if(have_posts()): ?><?php while(have_posts()):the_post(); ?>
<?php if (in_category(’15’)) continue; ?>

however – this messes up the number of posts that appear on each page. I have my Reading options set to display 10 posts per page, but my blog is displaying much less than that.

It appears that instead of removing posts of a certain category from the loop – this code is keeping them in, but just making them invisible.

Here is a link to my blog:
http://www.2nimprov.com/blog