Why a quick look misleads you
A single answer from each prompt is not enough to judge. Models have some randomness, so prompt A might get a great answer once and a poor one next time. Judge on one run each and you are half testing the prompt and half testing luck.
There is also a trap of taste. A prompt that reads nicely can still perform worse. To compare fairly, you have to look at the answers, not the prompts.
A fair comparison, in three rules
1. Same input
Feed both prompts the exact same input. If prompt A gets an easy example and prompt B gets a hard one, the test is rigged before it starts.
2. Same standard
Write down what a good answer needs before you look at any results: the tone, the length, the facts. Score both against that list, not against each other.
3. Several runs
Run each prompt three or four times. The one that stays good across all of them wins. A single strong answer does not settle it.
Consistency counts too
When you compare, do not only ask which prompt gave the best single answer. Ask which one gave good answers every time. A prompt that scores 8 out of 10 on every run is usually better to rely on than one that swings between a 10 and a 4.
Steady and good beats occasionally brilliant, especially for anything you plan to reuse.
A quick worked comparison
Say you want a prompt that writes product descriptions, and you have two versions. Here is the whole process in one pass.
Pick one product and feed it to both prompts. Before you read anything, write your standard: under 50 words, one clear benefit, no buzzwords. Run each prompt four times. Prompt A gives one great description and three flat ones. Prompt B gives four solid, on-standard descriptions every time. Prompt B wins, even though A produced the single best line. Steady and good beats occasionally brilliant, because you can rely on it.
That is the value of running each prompt several times. A single comparison would have told you A was best, and you would have been wrong.
Common ways a comparison goes wrong
A few habits quietly ruin a comparison and lead you to the wrong winner.
- → Different inputs. If each prompt gets a different example, you are testing the examples, not the prompts.
- → One run each. A single answer can be luck. You need a few runs to see the real pattern.
- → Moving the goalposts. Deciding what counts as good after you see the results lets you pick the prompt you already liked.
- → Judging the prompt, not the answers. A nicer-looking prompt can still produce worse output. Look at what comes out.
Keep the winner, note why
Once one prompt wins, write down what made it better. Maybe it was a clearer format line, or a tighter constraint. That note is worth more than the prompt itself, because it tells you what to do next time you write one from scratch.