Often, dealer pool water testing isn't as accurate as the testing you can do
yourself!
(It's worse than I had thought: see the note below.)
The reason?
Chlorine, chloramines, pH, and alkalinity levels in your swimming pool water can change significantly between pool and store. Plus, the complex test apparatus some dealers have is more subject to operator errors than a simple home pool kit -- especially when that operator is an briefly trained college student home for the summer. Swimming pool test strips, which have begun to be widely used, are too imprecise for the job, even when used correctly.
Test it yourself and save all those trips to your pool dealer! And, avoid all those "Oh, this might (but usually won't) help" chemical purchases.
Decent chlorine and pH test kits are less than $10. And for around $65 or so, you can perform ALL the swimming pool water tests that you might ever need! (testkit info)
By the way, don't be too impressed with the 'water analysis' computers many dealers have. The computer does NOT test your water. The college student, or whoever, does all the testing, and simply types the results into the computer. Worse, in many cases, the computer program is mismatched with the testing method, and will virtually always recommend chemical addition, even when your swimming pool is fine!
It's worth keeping in mind that swimming pool water analysis programs were developed for the marketing departments of swimming pool chemical companies! (And you wondered why you always get a printout recommending that you add more pool chemicals!)