Section 1: Animal Models

Section 2: Working with Mice

Mouse Life Spans
Mouse Strains
Raising Mice

Section 3: Planning Experiments

Experiment Design

Section 4: Evaluating Data

Average and Range
Graphing Choices
Interpreting Graphs

Section 5: Drawing Conclusions

Animal Models

Experiments that involve laboratory animal models are very important for increasing our understanding of conditions that affect humans. Animals that have been used as model systems include mice, rats, chickens, and goats. Animals are good models for humans because there are no basic differences between their physiology and human physiology: they all control their internal functions in about the same way and respond similarly to infection and injuries.

Using animal models for research is usually less expensive and less time-consuming than using humans. Researchers can also control experimental conditions (such as the amount of food or exercise) more easily for animals than for humans. There are fewer ethical concerns when using animals for research studies; however, researchers must follow strict guidelines for ethical treatment of animals.

Working with Mice—Mouse Life Spans


Mice are considered infants from birth to 21 days of age. At that age they are weaned (prevented from nursing)—their mothers are removed from the cages


Mice are considered juvenile (young) from 3 weeks to 8 weeks old.


After 2 months, mice are considered adults. Laboratory mice live about 1.5 years.

Working with Mice—Mouse Strains

Many different strains of mice are used in laboratory experiments. Each strain is composed of mice that are genetically identical. Researchers select the strain of mouse that is most appropriate for their work. This allows them to control the effect of genes on the results of their experiments.

The Animal Laboratory at the Energy Balance Institute has two strains of mice.

Strain A mice are brown. Strain B mice are white.

Working with Mice—Raising Mice


Mice are social animals that prefer to be housed together. At the Energy Balance Institute, the number of mice per cage is determined by the size of the mice and the size of the cage.

Food and Water

Mouse cages are checked daily. Water is provided in unlimited amounts. Food dishes are kept filled, unless experiment design calls for limiting the amount of food available.


Exercise wheels are not placed in mouse cages unless required by experiment design. When present, mice are allowed to use the wheels as determined by experiment design.

Planning Experiments—Hypothesis

A hypothesis is a testable statement that predicts a result.

For example:
Mice that have different genes will gain different amounts of weight in the first 8 weeks following birth.

Researchers may be able to make the hypothesis more specific. For example, if they know that gene X affects weight gain in mice, they might write the following hypothesis:

Mice that have gene X will gain more weight in the first 8 weeks following birth than mice that do not have this gene.

Planning Experiments—Experiment Design

Experimental groups, control groups, and growth conditions

Researchers identify experimental and control groups of animals based on their hypothesis.

For example, suppose the hypothesis is “Rats that exercise daily will gain less weight than rats that do not exercise.”

The experimental group would be rats that use an exercise wheel daily. The control group would be rats that do not use an exercise wheel, because exercise wheels are not provided for rats under standard laboratory conditions.

All other growth conditions should be the same for the two groups. They should receive the same amount of food and water, and they should be kept in cages of the same size with the same number of rats per cage.

Selecting the number of mice

Because the amount of weight gain or loss varies from individual to individual, all experiments conducted at the Energy Balance Institute use experimental and control groups with more than one mouse in each group.

However, raising mice and conducting experiments with them is expensive, so we do not use very large numbers of mice for every experiment.

Researchers at the Energy Balance Institute have decided that using 10 mice per condition is acceptable for our experiments.

Evaluating Data—Average and Range


Calculate the average by adding all of the measured values in a data set and dividing by the number of measures. For example, consider the following data:

The average number of daily calories consumed by this individual is 18,800 calories ÷ 7 days, or 2,686 calories.

The average of a set of measures evens out natural fluctuations, or variations, that often occur when measures are made across time or across individuals. The average provides researchers with an approximation of a “true” value for the measure.

Day of the week
Calories consumed per day
Sunday 2,200
Monday 3,200
Tuesday 2,500
Wednesday 2,400
Thursday 2,700
Friday 2,800
Saturday 3,000
Total 18,800
(Total ÷ 7)


The range for a data set gives the lowest and the highest values. For the individual in the previous example, calories per day ranged from 2,200 calories to 3,200 calories.

illustration of the concept “range” as a ladder with the lowest value on the bottom of the ladder, and the highest value at the top of the ladder

Reporting the range is useful to researchers because it indicates how much variation was observed in the measures.

Evaluating Data—Graphing Choices

Bar graphs

Bar graphs are used for data that can be sorted into categories. For example, nutrients fall into three major categories: carbohydrates, fats, and proteins. A bar graph would compare the number of calories of each nutrient consumed on different days

Line graphs

Line graphs are used for data that increase or decrease continuously. For example, an individual's height increases continuously across the first 16 to 20 years of life.

Interpreting Graphs

Graphs provide a picture of the data. Once researchers have prepared a graph, they can identify trends or patterns in the results of their experiments

For instance, a bar graph may show the number of calories from carbohydrates, fats, and proteins varying from day to day for an individual.

On the other hand, a line graph may show that human height increases throughout childhood and adolescence, and then levels off and remains constant in the adult years. How would the graph appear if humans continued growing throughout their lives?

Drawing Conclusions

Because the hypothesis and experiment are based on the research question, this question must be considered in drawing conclusions.

First, ask:
Do the results from the experiment provide an answer for the research question?

If the answer is "No" or "I don't know," the experiment, as it was designed, was probably inappropriate for the question. Think about the question and redesign the experiment.

If the answer is "Yes," ask:
Do the results support the hypothesis?

Whether the answer to this question is "Yes" or "No,” there is an answer for the research question. Use the evidence from the experiment to defend that answer.

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