Re: What is an experiment ?
Art Kendall wrote: [snip good points] in your quasi-experiment you can possibly contrast different levels of specific pollutants, as well as kinds of pollutants, in different rivers at different times. I'm not a biologist, but I would be amazed if temperature did not affect population sizes. Yeah, but would temperature also be related to pollultion levels, and if not, so what if it is related to the outcome under study. -Jay = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
Let me take a (somewhat) contrarian position to those previously expressed. An experiment is any test of a hypothesis. An experiment can involve the use of observational (unmanipulated) data, as long as the hypothesis is clearly stated prior to the collection of the data. While it is true that an experiment involving manipulation can provide some of the best evidence for causal relationships, causal relationships can be deduced from observation data combined with other information about how the world works. All of that said, the situation described below is what I would call a hypothesis-generating activity. That is, you want to look for a potential correlation that you will use to then test specific mechanisms (i.e., doex chemical X kill fish?). It would be a hypothesis-testing activity, if you had a prespecified hypothesis concerning a particular pollutant that previous experiments have shown to kill or otherwise harm fish. Sam Scheiner Voltolini wrote: Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? Thanks for any comments. Voltolini _ Prof. J. C. VOLTOLINI Grupo de Estudos em Ecologia de Mamiferos - ECOMAM Universidade de Taubate (UNITAU) Departamento de Biologia Taubate, SP, Brasil. CEP 12030-010 Tel: 0XX12-2254165 (lab.), 2254277 (secret. depto.) FAX: 12 - 2322947 E-Mail: [EMAIL PROTECTED] _ = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ = = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
SSCHEINE wrote: Let me take a (somewhat) contrarian position to those previously expressed. An experiment is any test of a hypothesis. An experiment can involve the use of observational (unmanipulated) data, as long as the hypothesis is clearly stated prior to the collection of the data. While it is true that an experiment involving manipulation can provide some of the best evidence for causal relationships, causal relationships can be deduced from observation data combined with other information about how the world works. I agree that you can test a hypothesis by using an observational study, but that does not make it an experiment. The original poster was looking for a definition to use in a lecture, and an experiment, by definition, involes assignment of treatments to experimental units. All of that said, the situation described below is what I would call a hypothesis-generating activity. That is, you want to look for a potential correlation that you will use to then test specific mechanisms (i.e., doex chemical X kill fish?). It would be a hypothesis-testing activity, if you had a prespecified hypothesis concerning a particular pollutant that previous experiments have shown to kill or otherwise harm fish. A study is hypothesis testing if the investigator is using it as such. Whether the particular study design would be expected to yield a valid answer is another matter. -Jay = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
What is an experiment ?
Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? Thanks for any comments. Voltolini _ Prof. J. C. VOLTOLINI Grupo de Estudos em Ecologia de Mamiferos - ECOMAM Universidade de Taubate (UNITAU) Departamento de Biologia Taubate, SP, Brasil. CEP 12030-010 Tel: 0XX12-2254165 (lab.), 2254277 (secret. depto.) FAX: 12 - 2322947 E-Mail: [EMAIL PROTECTED] _ = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
At 03:59 PM 2/20/02 -0300, Voltolini wrote: Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? the main issue is CONTROL OVER ... that the experimenter exerts over an independent variable if you want to compare diversity of fish ACROSS rivers ... and you find a difference, what does this necessarily have to do with contamination? i see three variables here ... diversity of fish ... rivers ... level of contamination (ie, where the gradients are different) what are you trying to show impacts on what? Dennis Roberts, 208 Cedar Bldg., University Park PA 16802 Emailto: [EMAIL PROTECTED] WWW: http://roberts.ed.psu.edu/users/droberts/drober~1.htm AC 8148632401 = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
What is an experiment ?
I think also that an experiment is the human attempt to make sense out of the chaotic world: the method is you assume chaos, H0 and then disprove it.. so you don't need controls because the experiment can be run to prove maybe that the equation for velocity is valid.. (validation experiment).. (ie you can disprove the null hypothesis, chaos by showing that somehthing always occurs).. I had an argument with a collegue: I said that B may not cause disease because there was no proof. They said that B did cause disease because there was no proof against it. Basically I think I'm write in that you always start with assuming chaos and no relation between disease and exposure and then you do your experiment.. no controls are needed, i think because sometimes you are just describing, as Jay pointed out.. like with testing the velocity equation, or discovering another relationship (equation etc.) = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
Voltolini wrote: Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? It's not an experiment, but not for the reason you are thinking. The reason it's not an experiment is because there has been no assignment of exposure or intervention to the units being investigated; rather, levels of the exposure (pollution) occur naturally among the groups being investigated (fish populations of each river). Hence, your study is not experimental, but rather, observational. As to the subject of control groups: there is no rule that says a control group has to have a zero level of the exposure. You can use, for instance, the river with the lowest pollution level as your control group (reference group might be a better name) and compare the effects of higher levels of pollution to that group. If you can quantify pollution levels, you could also model the effect of pollution level as a continuous variable in such a way that the lowest level of pollution in the study would implicitly be the reference level. The problem that you have in any observational study, however, is with drawing causal inferences from the results. Any observed association between pollution level and species diversity could at least logically be caused by any characteristic of the rivers that differed among them and that was associated with their pollution levels. If there are such extraneous differences, they must be controlled in some manner in order to make valid inferences about the effect of pollution on species diversity. -Jay = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
Voltolini wrote: Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? It's not an experiment, but not for the reason you are thinking. The reason it's not an experiment is because there has been no assignment of exposure or intervention to the units being investigated; rather, levels of the exposure (pollution) occur naturally among the groups being investigated (fish populations of each river). Hence, your study is not experimental, but rather, observational. As to the subject of control groups: there is no rule that says a control group has to have a zero level of the exposure. You can use, for instance, the river with the lowest pollution level as your control group (reference group might be a better name) and compare the effects of higher levels of pollution to that group. If you can quantify pollution levels, you could also model the effect of pollution level as a continuous variable in such a way that the lowest level of pollution in the study would implicitly be the reference level. The problem that you have in any observational study, however, is with drawing causal inferences from the results. Any observed association between pollution level and species diversity could at least logically be caused by any characteristic of the rivers that differed among them and that was associated with their pollution levels. If there are such extraneous differences, they must be controlled in some manner in order to make valid inferences about the effect of pollution on species diversity. -Jay = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
