Observational Studies and Experiments Ch. 10 Observational Studies and Experiments

Definitions: 1) Observational study - observe outcomes without imposing any treatment Retrospective studies collect data from the past Prospective studies identify subjects in advance and collect data as the events unfold 2) Experiment - actively impose some treatment in order to observe the response

100% of daily vitamins & essential oils! I’ve developed a new rabbit food, Hippity Hop. Makes fur soft & shiny! Hippity Hop Rabbit Food Increases energy! 100% of daily vitamins & essential oils!

Can I just make these claims? NO What must I do to make these claims? Do an experiment to prove it Who (what) should I test this on? Rabbits What do I test? The type of food

3)Experimental unit – the single individual (person, animal, plant, etc.) to which the different treatments are assigned 4) Factor – is the explanatory variable 5) Level – a specific value for the factor

6) Response variable – what you measure 7) Treatment – a specific experimental condition applied to the units

I plan to test my new rabbit food. What are my experimental units? What is my factor? What is the response variable? Rabbits Type of food How well they grow

8) Control group – a group that is used to compare the factor against; can be a placebo or the “old” or current item 9) Placebo – a “dummy” treatment that can have no physical effect

10) blinding - method used so that units do not know which treatment they are getting 11) double blind - neither the units nor the evaluator know which treatment a subject received

12) Confounding variable : the effect of the confounding variable on the response cannot be separated from the effects of the explanatory variable (factor)

(REQUIRED!) Principles of Experimental Design Control of effects of extraneous variables on the response – by comparing treatment groups to a control group (placebo or “old”) Replication of the experiment on many subjects to quantify the natural variation in the experiment Randomization – the use of chance to assign subjects to treatments

The ONLY way to show cause & effect is with a well-designed, well-controlled experiment!!

I’ll use my pet rabbit, Lucky! Hippity Hop Since Lucky’s coat is shinier & he has more energy, then Hippity Hop is a better rabbit food! Discuss the need for a comparison group.

Old Food Hippity Hop Now I’ll use Lucky & my friend’s rabbit, Flash. Lucky gets Hippity Hop food & Flash gets the old rabbit food. WOW! Lucky is bigger & shinier so Hippity Hop is better! Discuss the need for replication.

The Hippity Hop rabbits have scored higher so it’s the better food! Old Food Hippity Hop The first five rabbits that I catch will get Hippity Hop food and the remaining five will get the old food. Discuss the need for random assignment to treatment groups.

Place the numbers in a hat. Number the rabbits from 1 – 10. Old Food Hippity Hop The remaining rabbits get the old food. The first five numbers pulled from the hat will be the rabbits that get Hippity Hop food. 6 1 2 10 4 5 7 9 8 3 8 5 7 3 9 4 I evaluated the rabbits & found that the rabbits eating Hippity Hop are better than the old food! 1 5 7 Discuss the need for the evaluator to be blinded 10 3 2 6 8 9

Completely Random Design Group 1 5 Rabbits Treatment 1 Hippity Hop Food Compare Growth Random Allocation* 10 Rabbits Treatment 2 Placebo Group 2 5 Rabbits * Assign numbers to rabbits (01-10). Write each number on a slip of paper. Put all papers (make sure they are the same size) in a hat, mix up the slips and then draw numbers. The first 5 are assigned to the treatment group. The remainder were placed into the placebo group.

Hippity Hop Rabbit Food makes fur soft and shiny, & increases energy for ALL types of rabbits! Discuss scope of inference. Can I make this claim?

Experimental units? Factors? Levels? Response variable? Example: A farm-product manufacturer wants to determine if the yield of a crop is different when the soil is treated with three different types of fertilizers. Fifteen similar plots of land are planted with the same type of seed but are fertilized differently. At the end of the growing season, the mean yield from the sample plots is compared. Experimental units? Factors? Levels? Response variable? How many treatments? Plots of land Type of fertilizer Fertilizer types A, B, & C Yield of crop 3