The Scientific Method
What is a controlled experiment?
A controlled experiment contains two (or more) groups (or samples), which receive exactly the same treatment except for the alteration of a single variable, the experimental variable.
How is the scientific method an interplay between deductive and inductive reasoning?
As we collect observations of the world, we can begin to make general predictions (or perceptions) regarding phenomena. This process is known as inductive reasoning, making general predictions from specific phenomena. From these generalized perceptions of reality, specific predictions can be deduced using logic, generating hypotheses. Experimentation allows researchers to test the predictions of the hypotheses. As more and more similar but different experiments reinforce a specific prediction, growing support emerges for the development of a scientific theory, another example of inductive reasoning. In turn, a theory can assist in the development of additional, untested hypotheses using deductive reasoning.
Contrast descriptive and inferential statistics.
Descriptive statistics describes the nature of the data sampled observations, typically by central tendency and variation. Inferential statistics infer predictions of the larger population that the sample is based on through hypothesis testing.
What two aspects of a data set do descriptive statistics summarize?
Descriptive statistics summarize two aspects regarding the distribution of data: central tendency and variability.
How do experiments provide insight into causality?
Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a particular factor is manipulated. Controlled experiments support the causal nature of the treatment, by limiting variability of all variables except the experimental treatment.
How are inferential statistics used to conduct hypothesis testing after an experiment?
Hypothesis testing is a statistical inference that measures the relationships between the control and the experimental groups. The predictions of each hypothesis are compared to the observed phenomena and typically examined with statistical analysis. If the observed phenomena violate the predictions of the hypothesis, the hypothesis is said to be rejected. If observations do not violate the predictions of the hypothesis, the hypothesis is said to be supported.
How do natural experiments differ from controlled experiments.
In a controlled experiment, all factors are capable of being controlled with the alteration of a single variable, the experimental treatment. A natural experiment does not have ultimate control over all factors of the experimental setup.
What is a natural experiment?
In a natural experiment, researchers rely on observations of replicates exposed to a variety experimental and control conditions, and infer an effect. The experimental design of natural experiments seeks to select replicates that closely resemble each other as possible, but vary in preferably one factor.
Contrast pure causation, reverse causation , common causation, cyclic causation, indirect caustion, and coincidence.
In pure causation, x causes y. Heat increases chemical reaction rates. In reverse causation, y causes x. Chemical reaction rates increase temperature. In common causation, x and y are affected by a third variable. For example, ice cream sales and drowning deaths are caused by a third variable, temperature. In cyclic causation, x causes y and y causes x. In this manner, there is feedback, either positive or negative. Predator/prey relationships typically follow cyclic causation. Indirect causation happens, when x is correlated with y, but the effect on y is directly affected by another variable, z, which in turn is affected by x. Predator abundance is caused by herbivore abundance which is affected by plant abundance. So predator abundance is also correlated with plant abundance, but indirectly through herbivore abundance. In a coincidence, two variables are correlated, but there is no causal association between them.
How is standard deviation related to statistical confidence?
Low standard deviation indicates a high confidence that the measured sample represents the population as a whole. For example, if you measured the height of several random people would that represent the variability in height for the whole human race? Standard deviation is inversely proportional to the sample size. More measurements of height provide a smaller standard deviation.
With respect to standard deviation, why do experiments require many replicates?
Scientific experiments ideally have many replicates to minimize the effect on standard deviation based on small sample sizes. Standard deviation is inversely proportional to the number of replicates. If the standard deviation is acceptably low, this tells us that the sample mean is very close to the population mean.
How does standard deviation define a data set?
Standard deviation is the most common way statistical variability is calculated. A low standard deviation indicates a small variation in the data from the mean, where a high standard deviation indicates data are spread across a large range of values.
What is statistical confidence?
Statistical confidence describes the reliability of a sampled population.
Contrast the control group and and experimental groups .
The control group is a random subset of the subjects (or units) to be examined that either do not receive the experimental treatment, or receive the standard treatment. The experimental group is a random subset of the subjects (or units) to be examined that does receive the experimental treatment. Typically, an experiment will have many experimental groups to examine the range of effects of the treatment.
What is the difference between dependent and independent variables.
The dependent variable is expected to change based on the presence, absence or abundance of the experimental treatment. The independent variable is a component of the experiment where the inputs are purposefully manipulated. The independent variable is a component of the experiment where the inputs are purposefully manipulated.
Contrast null and alternative hypotheses.
The null hypothesis indicates there is no relationship between two measured phenomena or no association among groups. An alternative hypothesis indicates that there is some relationship between two measured phenomena or association among groups. Typically, this means the experimental treatment has a measurable effect.
What is deductive reasoning or logical deduction?
a top-down logical methodology in which a conclusion is reached reductively by applying general rules that hold over the entirety of a closed domain of discourse, narrowing the range under consideration until one or more specific conclusions remain.
What is the role of empiricism in the scientific method?
all hypotheses and theories must be tested against observations of the natural world rather than resting solely on reasoning and intuition.
What is an experimental replicate?
A replicate is the repetition of an experimental condition on different subjects (or units, more generally), in order to measure and account the variability within the groups examined. Furthermore, statistics assumes replicates are representative of the population being studied, and preferably randomly selected.
Describe three ways central tendency is measured.
1. Mean = the calculated middle of the data set. <br> 2. Median = the observed middle of the data set <br> 3. Mode = the most common value in a data set.
What are the components of a good scientific question?
A scientific question is: simple, measurable, testable, answerable, and specific in scope.
What is the benefit of a blind experiment?
A blind experiment is an experiment in which information about the test is masked (kept) from the participant and/or experiment administrator, to reduce or eliminate bias, until after a trial outcome is known. If both tester and subject are blinded, the trial is called a double-blind experiment.
In general, what is a scientific hypothesis?
A hypothesis is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous observations that cannot satisfactorily be explained with the available scientific theories. Even though the words hypothesis and theory are often used synonymously, a scientific hypothesis is not the same as a scientific theory.
What is the purpose of a placebo in a controlled experiment?
A placebo is a simulated or otherwise ineffectual treatment given to a control group. A subject given an ineffectual treatment will often have a perceived or actual improvement in their condition, a phenomenon commonly called the placebo effect. A placebo is also known as a negative control.
What is experimental treatment (or experimental variable)?
A treatment (also called the experimental variable) is the variable that scientists alter within an experiment. For example, if a study was comparing the effect of sugar consumption on exam scores, the experimental variable would be the sugar. It is the component of the experiment that will differ between the groups.
What is the purpose of a scientific experiment?
An experiment is a procedure carried out to support, refute, or validate a hypothesis
What is the purpose of a negative control and a positive control in a controlled experiment
Negative controls are groups where no phenomenon is expected. They ensure that there is no effect when there should be no effect. In drug testing, a negative control is a group that has not been administered the drug of interest. This group receives a placebo. The negative control group should show a null effect. A positive control is a known effect from the experimental treatment, used to confirm to validity of the measurements acquired. For example, if you were testing sugar concentrations in food, you could ensure the experiment is working by measuring a substance with a known sugar concentration.
In statistics, what is variability?
Variability is a measured deviation of data from the central tendency.
What is inductive reasoning?
derives general principles from specific observations, in which the observations are viewed as supplying evidence for the truth of the more general conclusion. The philosophical definition of inductive reasoning is more nuanced than simple progression from particular/individual instances to broader generalizations. Rather, the premises of an inductive logical argument indicate some degree of support (inductive probability) for the conclusion but do not entail it; that is, they suggest truth but do not ensure it.