You sort of develop experiments as you go along, improving things and getting better and better data. Once you have enough good data you can publish it 🙂 So in a way all these early measurements are practice for you, as you develop your technique and the apparatus and your understanding.
Quite often if you are showing an experiment to someone it is because you are trying to teach them how to do it, so often you use nice reliable samples (i.e. a good, simple, setup) to teach with so they can develop their techniques without trying the impossible at first!
If its the first time I am doing an experiment I will practice it (or optimize it) on tissue or cells that was have lots of before I run the experiment on tissue or cells which have limited supplies. This is to save on tissue/cells in case it goes wrong. Once I know that it is all working then I will repeat the experiment and then I often have to do the experiment several times on the tissue/cells that I want to make sure the results are reliable and ‘real’ as sometimes they vary from experiment to experiment.
Most scientists get experiments wrong at first but this is normal. The only real problem in getting an experiment wrong/practicing is the cost of the experiment or a limited number or cells. So as long as you make sure you have some practice cells or make sure the experiment isn’t costing £10k then its great to practice.
For me, sometimes if I am trying a new method, I will do a small batch of samples first to make sure that I can get it working the way I want it to. This is a ‘safe’ way of doing an experiment- more for financial reasons. The types of experiment I do are often expensive but generate a tonne of data. These experiments can take over a week to perform and involve a number of steps; because of the increased number of steps it’s often easy to make a mistake without you realising. If you order in and do a monster experiment with all your samples at once (say 100), and you stuff up, that’s gonna cost a lot. If you run only a small number of samples first (say 5) and it stuffs up, you can find out where you stuffed up and make sure to correct it before running the remaining samples (95). It makes it a lot easy to break the news to the boss as well if you do stuff up (5 vs 100) 🙂
practising things before showing them is very important in science, we all make mistakes (i make loads in my work) but the important thing is to be confident we’ve got it right before showing it (to my boss or your teacher say) and in science, the most important thing is to make sure it’s right before you publish it, otherwise it’s very embarrassing 🙂