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Variables In a scientific experiment, you are measuring how changing one factor in a system affects another factor. These factors are called variables, and an experiment generally has three types: independent, dependent and controlled. When choosing your variables, keep in mind that good variables are usually those that you can measure with a number (e.g., concentrations of pollutants, time of day, temperature) or observe objectively (e.g., colors, growth). The independent variable is the one that you, as the scientist, change or manipulate. Your experiment should have only one independent variable. For your EnvironMentors project, possible independent variables could be time (e.g., testing the amount of pesticides in the water at the same site on different days) or location (e.g., measuring noise pollution at different locations at the same time of day). Time and location are just two examples of independent variables. You and your mentor should work together to determine the best independent variable for your project. The dependent variable changes in a consistent manner in response to changes you make in the independent variable. In the examples above which used time and location as the independent variables, the dependent variables would be the amount of pesticides in the water, and the amount of noise pollution, respectively. Whereas there should only be one independent variable, the number of dependent variables can vary. For instance, if you were investigating water quality at the same site on different days, your independent variable would be time, and each different pesticide concentration you measure in the water would be a different dependent variable. The controlled variables are those that you try to keep constant throughout your experiment. These are all variables that might affect your dependent variable. If you were measuring the concentrations of pesticides in water on different days, you would want to make sure that things like the weather and outside temperature remain relatively constant. You should keep track of your controlled variables as carefully as your other variables. Remember that it is important to try to keep your controlled variables as constant as possible. For instance, if you measured the noise pollution at two different traffic intersections in your city (independent variable = location; dependent variable = noise pollution) but made your measurements at different times of day, you wouldn’t be able to tell if changes in the amount of noise pollution were due to the different location or the different time of day. Most experiments will have more than one controlled variable. Talk to your mentor and make a list of all the variables you think might affect your dependent variable, and the best way to make sure you keep those variables constant. You must also compare an experimental group to a control group. The control group is the set of experimental trials where the independent variable is set at its natural state. An experimental group is the set of experimental trials where you change the independent variable. Example: In an experiment investigating the effects of acid rain on plant growth, you might grow different plants and water them with water at different pH values. The plants watered with water with a pH natural� rain value of 5.6 would be your control group and the plants watered with water with more acidic pH values would be your experimental group. In some projects, it might not be possible to measure a control group. Be sure to discuss this with your mentor and/or chapter coordinator

Developing an Experimental Procedure After identifying your variables, the next step is to design a procedure for how you will manipulate your independent variable, and how you will measure the resulting changes in your dependent variable(s). Each time you perform your experiment it is called a trial. Remember that for your EnvironMentors project, you will need to conduct at least three trials of data collection. Three trials are necessary to collect sufficient data to answer your research question. Conducting more than three trials is strongly recommended as additional trials will produce additional data and therefore reduce the margin of error. Tips for Developing an Experimental Procedure  In order to make sure that the changes you observe in your dependent variable are due to changes in your independent variable, you should make sure that your controlled variables remain constant.  You must also compare an experimental group to a control group. Know the difference between controlled variables and your control group.  Develop an experimental procedure by writing a step-by-step list of how you will conduct your experiment. Be as detailed as possible. When writing your procedure, pretend that another scientist will be repeating your experiment. They should be able to perform your experiment exactly as you did by following your step- by- step procedure.  Your experimental procedure should detail how you plan to execute Effective Data Sampling. Experiment Design and Effective Data Sampling Several attributes characterize effective sampling: accuracy, consistency, and persistence. You will see these tips again in the next section Data Collection & Analysis. Accuracy is the foundation of all scientific observation. Care in taking the measurements is the first step. The equipment used and efforts to keep it in good condition are very important. Strive for perfection in recording data entries and tests. Consistency means doing the experiment the same way every time. This lets you compare your data against others doing the same tests. Consistency is also important over time. You want to watch changes and trends in your data; the data that you have taken in the past must be directly comparable to the data you have taken today. Persistence means regular and frequent observations, allowing a greater understanding of what is happening at a measurement site. Also, regular observations are easier to interpret and can be used with greater confidence, especially when unusual phenomena are being measured.

Creating a Materials List What type of supplies and equipment will you need to complete your EnvironMentors project? By making a complete list ahead of time, you can make sure that you have everything on hand when you need it. Some items may take time to obtain, so making a materials list in advance is key! As with your Experimental Procedure, your Materials List include the specific amounts of each item you’ll need (500 g of soil, rather than just listing soil). Ask yourself two questions when developing your materials list: Have you listed all necessary materials? Have you described the materials in sufficient detail?  Tip: At a simplified level, you can think of your Materials as the ingredients list of a recipe. A recipe that lists only flour, sugar and eggs would be useless without specified quantities. Your materials list needs to be as detailed and specific as a cooking recipe ingredients list. Science provides an example of a “good” and a “bad” materials list  A Good Materials List Is Very Specific

X A Bad Materials List

500 ml of de-ionized water


Stopwatch with 0.1 sec accuracy


AA alkaline battery


If you and your mentor have any problems obtaining the materials and equipment you need for your experiment, be sure to talk to your chapter coordinator.