Substituting yarns is something that can really tangle you up in knots (I couldn't resist). There are many ways to figure out what you need to make up a certain pattern and come away with something that will give you a great finished project. Here's one process of elimination that might help you get through your decision. I'm using a knit example here, but I think it would work pretty well with crochet also.
Let us imagine... Obsessive Knitter A. was flipping through the May 2008 issue of Creative Knitting, hot off the press, and this cardigan caught her eye: It's simple, but not too boring - she loves the 3/4 sleeves and the slightly sloping V-neck. Maybe her daughter would like a basic cardigan to keep her warm while up late studying for finals! So A. checks out the yarn in the pattern:
She notes that the pattern has generous sizing - the smallest cardi is 36" and it goes all the way up to 52". The sample was knit in a yarn that isn't available at Sage (her LYS) and she browses around the shop to see what else would do.
Picking out a new color - blue! - A. pulls all the blue "worsted weight" yarns she can find and piles them on the table. I see her growing pile and ask about her project. "How am I going to pick from all these?" she asks. "Would you like a summer cardigan or a fall/winter/spring cardigan?" is usually my first question. "Well, my daughter is always cold and she would get more use out of a fall/winter sweater."
So we put the cottons away and take a look at the wools:
Wool is warm, alpaca and llama are warmer, tweed is tweedy, malabrigo is oh-so-soft. What do we do now?
"Will your daughter bring it home to be cleaned?"
We are left with superwash wools and wool blends:From here A.'s decision is personal - how much money does she want to spend?
What would she like to be touching while she is knitting?
Which shade of blue is just right?
In the end, A. picks Pure Merino from Berroco. She says, "Well, the superwash wool seems a bit scratchy... Encore is nice, I've used it before, but I the Pure Merino is just so squishy and soft. I'll just have to tell her how much time and money I spent on it so she doesn't leave it at a friend's place!"
In the span of a short trip, A. has at least narrowed her choices down to 4 very nice yarns. She can write down some names and go home to think about it for a bit if she wants, or make a decision in the store.
If you need any help calculating yardage, making a pile of yarns that fit your gauge or ANYthing else, just let us know! Sometimes, we even hand out more advice than you actually needed (we don't shut up)...
Have fun with the string,