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  1. Producing Data: Experiments BPS chapter 9 © 2006 W.H. Freeman and Company

  2. Explanatory variables Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. What is the explanatory variable? • The amount of scent. • Presence or absence of the floral scent. • Time to complete the pencil and paper mazes. • Whether the subject was able to complete the mazes quicker while wearing the floral-scented mask.

  3. Explanatory variables (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. What is the explanatory variable? • The amount of scent. • Presence or absence of the floral scent. • Time to complete the pencil and paper mazes. • Whether the subject was able to complete the mazes quicker while wearing the floral-scented mask.

  4. Response variables Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. What is the response variable? • The amount of scent. • Presence or absence of the floral scent. • Time to complete the pencil and paper mazes. • Whether the subject was able to complete the mazes quicker while wearing the floral-scented mask.

  5. Response variables (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. What is the response variable? • The amount of scent. • Presence or absence of the floral scent. • Time to complete the pencil and paper mazes. • Whether the subject was able to complete the mazes quicker while wearing the floral-scented mask.

  6. Individuals Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. What are the individuals? • The masks (floral-scented or unscented). • The 22 subjects. • The mazes.

  7. Individuals (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. What are the individuals? • The masks (floral-scented or unscented). • The 22 subjects. • The mazes.

  8. Control Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The researchers incorporated control/comparison by • Giving each subject a floral-scented and an unscented mask. • Randomly assigning half of the subjects to wear a floral-scented mask only and the other half to wear the unscented mask only. • Giving each subject two sets of mazes.

  9. Control (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The researchers incorporated control/comparison by • Giving each subject a floral-scented and an unscented mask. • Randomly assigning half of the subjects to wear a floral-scented mask only and the other half to wear the unscented mask only. • Giving each subject two sets of mazes.

  10. Randomization Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The researchers incorporated randomization by • Randomly selecting the subjects to participate in the study. • Randomly assigning half of the subjects to wear the floral-scented mask and the other half to wear an unscented mask. • Randomly assigning the order that each subject receives the floral-scented and unscented masks.

  11. Randomization (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The researchers incorporated randomization by • Randomly selecting the subjects to participate in the study. • Randomly assigning half of the subjects to wear the floral-scented mask and the other half to wear an unscented mask. • Randomly assigning the order that each subject receives the floral-scented and unscented masks.

  12. Replication Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The researchers incorporated replication by • Using two masks. • Using two sets of mazes. • Using three mazes within each set. • Using twenty-two subjects. • Repeating the entire experiment a second time.

  13. Replication (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The researchers incorporated replication by • Using two masks. • Using two sets of mazes. • Using three mazes within each set. • Using twenty-two subjects. • Repeating the entire experiment a second time.

  14. Variables Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. If age affects whether the presence of a floral scent improves learning ability and was not included among the variables studied in the experiment, then age is • An explanatory variable. • A response variable. • A lurking variable. • Confounded with floral scent.

  15. Variables (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. If age affects whether the presence of a floral scent improves learning ability and was not included among the variables studied in the experiment, then age is • An explanatory variable. • A response variable. • A lurking variable. • Confounded with floral scent.

  16. Experimental design Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The experimental design used is called a • Completely randomized design. • Randomized block design. • Matched pairs design.

  17. Experimental design (answer) Hirsch and Johnston from the Smell & Taste Treatment and Research Foundation in Chicago believe that the presence of floral scent can improve a person’s learning ability in some situations. To test this hypothesis, they set up an experiment in which each of 22 subjects completed 2 sets of three pencil and paper mazes, one set while wearing a floral-scented mask. Each subject wore a floral-scented mask and an unscented mask, and the order was randomized. The researchers measured the length of time it took each subject to complete the sets of mazes. The experimental design used is called a • Completely randomized design. • Randomized block design. • Matched pairs design.

  18. Statistical significance If there is a statistically significant difference between the average times to complete the mazes while wearing the floral-scented mask and the unscented mask, then the difference in average times to complete the mazes between the floral-scented mask and the unscented mask is • Too large to be due to chance alone. • Too small to be due to chance alone. • So large that we can reasonably attribute it to chance. • So small that it is likely due to chance.

  19. Statistical significance (answer) If there is a statistically significant difference between the average times to complete the mazes while wearing the floral-scented mask and the unscented mask, then the difference in average times to complete the mazes between the floral-scented mask and the unscented mask is • Too large to be due to chance alone. • Too small to be due to chance alone. • So large that we can reasonably attribute it to chance. • So small that it is likely due to chance.

  20. Experimental design An Austrian study investigated whether maintaining a surgery patient’s body temperature close to normal by heating the patient during surgery decreases wound infection rates. Patients included in the study were undergoing colon or rectal surgery and were randomly assigned to one of two treatment groups. In the normalthermic group, patients’ core temperatures were maintained near normal 36.5 degrees Celsius. In the hypothermic group, patients’ core temperatures were allowed to decrease to about 34.5 degrees Celsius. The design is called a • Completely randomized design • Randomized block design • Matched pairs design

  21. Experimental design (answer) An Austrian study investigated whether maintaining a surgery patient’s body temperature close to normal by heating the patient during surgery decreases wound infection rates. Patients included in the study were undergoing colon or rectal surgery and were randomly assigned to one of two treatment groups. In the normalthermic group, patients’ core temperatures were maintained near normal 36.5 degrees Celsius. In the hypothermic group, patients’ core temperatures were allowed to decrease to about 34.5 degrees Celsius. The design is called a • Completely randomized design • Randomized block design • Matched pairs design

  22. Experimental design In the previous experiment involving patients’ temperatures, both men and women were the patients. If the men and women were separately assigned to treatments, the design would be a • Completely randomized design • Randomized block design • Matched pairs design

  23. Experimental design (answer) In the previous experiment involving patients’ temperatures, both men and women were the patients. If the men and women were separately assigned to treatments, the design would be a • Completely randomized design • Randomized block design • Matched pairs design

  24. Randomized block design In a randomized block design, a block contains • Individuals that are similar with respect to the characteristic that defines the block. • Individuals that are assigned to the same treatment. • Individuals that are similar with respect to the characteristic that defines the block and that are assigned to the same treatment.

  25. Randomized block design (answer) In a randomized block design, a block contains • Individuals that are similar with respect to the characteristic that defines the block. • Individuals that are assigned to the same treatment. • Individuals that are similar with respect to the characteristic that defines the block and that are assigned to the same treatment.

  26. Randomized block design In a randomized block design • All of the subjects are allocated at random among the treatments. • The random assignment of individuals to treatments is carried out separately within each block. • All the individuals within one block are randomly assigned one treatment.

  27. Randomized block design (answer) In a randomized block design • All of the subjects are allocated at random among the treatments. • The random assignment of individuals to treatments is carried out separately within each block. • All the individuals within one block are randomly assigned one treatment.

  28. Experimental design True or false: An advantage of the randomized block design (RBD) over the completely randomized design (CRD) is that the RBD eliminates confounding between a lurking variable (blocking variable) and the response variable. • True • False

  29. Experimental design (answer) True or false: An advantage of the randomized block design (RBD) over the completely randomized design (CRD) is that the RBD eliminates confounding between a lurking variable (blocking variable) and the response variable. • True • False

  30. Experimental design True or false: An advantage of the randomized block design (RBD) over the completely randomized design (CRD) is that the RBD eliminates the variation associated with the blocking variable and makes detection of statistical significance easier. • True • False

  31. Experimental design (answer) True or false: An advantage of the randomized block design (RBD) over the completely randomized design (CRD) is that the RBD eliminates the variation associated with the blocking variable and makes detection of statistical significance easier. • True • False

  32. Problems with experiments A study claims that patients who receive surgery for intestinal cancer live much longer after treatment than patients who are treated without surgery. However, doctors operated only on patients in relatively good condition so we cannot conclude from this study that surgery lengthens intestinal cancer patients’ lives. This is an example of • Confounding. • A lurking variable. • A double-blind experiment. • The placebo effect.

  33. Problems with experiments (answer) A study claims that patients who receive surgery for intestinal cancer live much longer after treatment than patients who are treated without surgery. However, doctors operated only on patients in relatively good condition so we cannot conclude from this study that surgery lengthens intestinal cancer patients’ lives. This is an example of • Confounding. • A lurking variable. • A double-blind experiment. • The placebo effect.

  34. Double-blind experiments Medical experiments are often double-blind. This means that • All individual data are kept confidential. • Neither the subject nor the doctor/administrator knows which treatment the subject receives. • Doctors are not allowed to decide which treatment a patient will receive; subjects are randomly assigned to treatments. • The subjects in the control group receive a placebo treatment.

  35. Double-blind experiments (answer) Medical experiments are often double-blind. This means that • All individual data are kept confidential. • Neither the subject nor the doctor/administrator knows which treatment the subject receives. • Doctors are not allowed to decide which treatment a patient will receive; subjects are randomly assigned to treatments. • The subjects in the control group receive a placebo treatment.

  36. Experiments An advantage of experiments over observational studies is • An experiment can provide evidence of cause and effect. • An experiment can compare two or more groups. • An experiment can include explanatory and response variables.

  37. Experiments (answer) An advantage of experiments over observational studies is • An experiment can provide evidence of cause and effect. • An experiment can compare two or more groups. • An experiment can include explanatory and response variables.

  38. Experiments Which of the following principles of good experimentation does an observational study not incorporate? • Control or comparison • Random assignment to treatments • Replication

  39. Experiments (answer) Which of the following principles of good experimentation does an observational study not incorporate? • Control or comparison • Random assignment to treatments • Replication

  40. Experiment vs. observational study The Minnesota Study of Twins Reared Apart investigated whether similar attributes of twins are related to heredity or the influence of family environment. The researchers conducted an intense psychological and physiological assessment of 56 pairs of identical twins reared apart and several hundred identical twins reared together and compared the similarities of the twins in both groups. Is this study an experiment or an observational study? • Experiment • Observational study

  41. Experiment vs. observational study (answer) The Minnesota Study of Twins Reared Apart investigated whether similar attributes of twins are related to heredity or the influence of family environment. The researchers conducted an intense psychological and physiological assessment of 56 pairs of identical twins reared apart and several hundred identical twins reared together and compared the similarities of the twins in both groups. Is this study an experiment or an observational study? • Experiment • Observational study