A hypothesis may lead to experiment and both may ..

You can remedy the problem of too few independent units by using more classes. For example, you might have 10 classes in one group and 10 classes in the other group. However, violation of independence is only one problem with using intact groups. Using intact groups exposes your study to three serious threats to construct validity: demoralization, compensation, and diffusion of treatment.

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Some field experimenters try to regain the power lost due to having high levels of random error and insensitive measures by using a large number of participants. To get large numbers of participants, some researchers do field experiments on intact groups. For example, they might use a few large classes or a work group.

A Field Experiment in Helping: By David Watson - Google Docs

Usually, losing more participants from one group than the other is due to one of two reasons. First, the treatment is too intense. In such cases, the treatment should be toned down or eliminated. To use a manipulation that leads to such a high drop-out rate is often unethical. To take an extreme case of using an unethical level of treatment, suppose the television these children were watching was X-rated violence. In that case, mortality from the treatment group would be high (although we would hope that an ethics committee would prevent such a study from being conducted). Second, mortality from the treatment group will be higher than from the control group if the control group is left alone to engage in their normal activities. For example, if the experimental group was to watch a prescribed set of programs at home whereas the control group was simply allowed to do whatever they normally did at home, mortality would be higher in the experimental group. Therefore, the control group should always get some kind of treatment, even a placebo treatment.

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To imagine the difficulties of random assignment in the field, suppose you wanted to study the effects of television violence on children's behavior. You approach parents and tell them that you want some children to watch certain non-violent television programs (e.g., Mr. Rogers, Sesame Street) and other children to watch violent television programs, such as "TV wrestling", boxing, and violent action/adventure shows. You may find that few parents will let you randomly assign their child to either condition. If you say, "I want to be able to assign your child to either one of these conditions," many parents will object. Some will say, "You can show my child Sesame Street, but you're not going to make my kid watch violence and trash!" Other parents will say, "You can make my kid watch wrestling. I watch it all the time anyway. But not those other shows. They're on the same time as my shows. You're not going to make me sit around and watch childish junk!" However, the hassles with the parents may be nothing compared with the hassles of getting the children themselves to agree to random assignment. Yet, with enough persistence (and enough money), you could probably get people to agree to random assignment. But once you've done that, you face a huge problem: How do you know that participants will watch the television shows you assigned? You cannot go to everyone's house. You cannot trust young children to carry out your instructions. You cannot trust parents to supervise the children because they may be busy with other tasks. The children are too young to be trusted to implement the treatment program. Therefore, the prospect of using random assignment to determine children's television diets seems intimidating. In fact, the idea of randomly assigning children to television-viewing seems so intimidating that most investigators researching the effects of TV have often avoided field experiments. This is unfortunate because such experiments would provide the strongest evidence about the effects of viewing violent television shows. Have these researchers given up too soon? Cook and Campbell (1976) claim that researchers often give up on random assignment faster than they should. Cook and Campbell argue that random assignment can often be used in the field – if the researcher is creative. In the case of researching the impact of television on children's behavior, researchers may have given up too soon. Perhaps researchers should approach a nursery school. If the nursery school would cooperate and get informed consent from the parents and children, the television-viewing could take place at the school as part of the children's ordinary routine. In this way, you would know that participants were getting the treatment they were assigned to.
Unfortunately, even after you assign your participants to condition, they may not stay assigned. Mortality may raise its ugly head. That is, participants may drop out of your experiment before you collect the dependent measure. For example, suppose that you are doing the television violence experiment with nursery school children. As the study progresses, you find that participants are dropping out of the violent television condition (perhaps the kids are getting too violent or the parents are having second thoughts). However, participants are not dropping out of the non-violent condition. The fact that participants in one group are more likely to quit than participants in the other group threatens the study's internal validity. That is, if the violent television group is more aggressive, we cannot say whether this is due to the less aggressive children dropping out of the violent television group or to television violence causing children to be aggressive.