What does it mean to compare? Really, what exactly are you doing when you compare? Yes, the obvious - you're seeing if A is better or worse than B. You're deciding which.
And for the why? Because you either want high quality, a good price, or both.
When and where, of course is once, twice, or a whole bunch of times.
What? For the sake of where we are, let's say you're comparing baby carriers.
How? Now, that's the real question. The how gives the details that matter. How do you compare baby carriers? Think about it. What are you thinking about when you compare?
You look at price and safety, certainly. You may jiggle it around a bit, give some tugs and squeezes, feel the fabric, maybe even put it on if it's a backpack, front pack, or sling. You may ask questions, read some of the instructions or safety warnings. You're absorbing details with your senses.
Now, you might be thinking – ookkaayy …
Watch people shop - most people will hear and see the details, fewer will notice the smell and feel, and unless you're comparing foods, a measly few will throw in taste. (In the case of babies, you might actually want to taste because they definitely will. Just wash it when you get home – someone else may have done that, too.)
The point is, how many details are you really paying attention to? And how many of those details are you visualizing? For some people, this may seem like a fairly stupid question. These people are the natural visualizers. For many, however, images don't play a big part in detail gathering.
What if you are buying a baby carrier online? You can't use all your senses. Your eyes, maybe your ears if there's some kind of video, but you can't smell, taste, or touch. You have to imagine the baby carrier based on the information you're getting. Makes sense to get as much information as possible.