If the new drug doesnt work we can rule out a problem with our equipment by

# If the new drug doesnt work we can rule out a problem

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properly. If the new drug doesn’t work, we can rule out a problem with our equipment by showing that the positive control drug works. The “negative-control” sets what we sometimes call the “baseline”. Suppose we are testing a new drug to kill bacteria (an antibiotic) and to do this we are going to count the number of bacteria that are still alive in a test tube after we add the drug. We could set up an experiment with three tubes. One tube could contain the drug we want to test. The second tube would contain our positive control (a different drug which we know will kill the bacteria) The last tube is our negative control – it contains a drug which we know has no effect
on the bacteria. This tells us how many bacteria would be alive if we didn’t kill any of them. If the new drug is working, there should be fewer cells left alive in the first tube compared to the last tube and ideally then number of cells still alive (if any) should be the same in the first and second tube. So “controls” are important to scientists because it helps us validate the performance of our experimental set- up and tells us what effects we can reasonably expect to observe. CHECK FOR UNDERSTANDING Tell whether the control group is positive or negative control. a. An investigator studies the amount of alcohol produced by two groups of yeasts when one group is incubated in the presence of sugar, and one group in the absence of sugar. Positive or Negative Control: Negative b. The effect of artificial light on photosynthesis is measured by collecting oxygen produced by a group of plants exposed to an artificial light, and a group of plants exposed to natural sunlight. Positive or Negative Control: Positive STEP 7: DETERMINING REPLICATION Another essential aspect of experimental design is replication. Replicating the experiment means that the scientist repeats the experiment numerous times using exactly the same conditions to see if the results are consistent. Being able to replicate a result increases our confidence in it. However, we shouldn't expect to get exactly the same answer each time, because a certain amount of variation is normal in biological systems. Replicating the experiment lets us see how much variation there is and obtain an average result from different trials. Part II: For each of the statements identify the variables, control group, and experimental group. In the control group determine whether your control group is positive or negative control. In the experimental group, make sure to identify the constants. 1. Eating breakfast increases performance in school. Independent Variable: Eating breakfast Dependent Variable: Performance in school Control Group; type of control: Students who don’t eat breakfast, negative Experimental Group w/ Constants: Students who eat breakfast 2. Salt in soil affects plant growth. Independent Variable: Salt in soil Dependent Variable: Plant growth Control Group; type of control: Positive
Experimental Group w/ Constants: Amount of salt, amount of soil, type of salt, type of plant, type of soil Part III: Write a hypothesis for each of the following problem statements. Identify the dependent and independent variable for each. **Remember a hypothesis is an IF (cause or IV)...THEN (effect or DV)

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• Fall '16
• investigator, Experimental Group, Scientific control

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