FAQ for Sample Size

I received a question via e-mail that will be helpful to many students so I'm answering it here also.

If you are doing plants do you need a lot of plants?

Let me say it this way...

In order to develop a solid conclusion, one must have lots of data. This can be accomplished in one of two ways.

1. Repeated Trial: This is when the same test is performed over and over. Think about Drops on a Penney. In that experiment you tested your independent variable at least 2 times. This is how you will get data for experiments where it's easy to repeat the test (rolling a ball, dropping something, cleaning up oil, chromatography to identify something and so on.
2. Multiple Subjects in a Group: This is where plants fall. Due to the length of time that it takes plants to grow it is nearly impossible to grow one plant to observe it and then start again and again and again. So, you grow many, many plants. How many? That depends on several factors. If you are testing something on plants that you buy, the stronges limiting factor is your budget. However, the minumum number of plants in a group should be between 5 and 25. Twenty five is not an unreasonable number if your plants are small. BUT that's 25 in EACH group: the control group and each test group.

If you grow plants from seeds then you can probably plan that the groups should have a minimum of 25 plants, but 100 would be ideal. After all, not all seeds sprout. Read the germination data on the package.

Is this a lot of seeds and plants??? Yes, but if you want to have a valid, data based conclusion you will need to have lots of data.

Other limiting factors besides your budget include the space that your project will take up, the time of year in which you are starting your project, and how fast your plants grow.

Can you do a plant project? Sure you can, but please make sure that the project you choose is doable for your time, expertise, budget, and how much room you have at home. And please don't put soda on plants unless you know why you are doing this.