Bio 120 LAB #2- SCIENTIFIC INVESTIGATION

Lab Objectives

After completing this lab topic, you should be able to:

1. ask scientific questions and develop hypotheses.

2. identify and describe the components of a scientific experiment.

3. design an experiment.

Questions and Hypotheses

All scientific investigations start with a question. However, not all questions can be answered scientifically. Which of the following questions can be answered scientifically?

1. Do humans communicate by chemical signals called pheromones?

2. Is the use of antibacterial soaps and cleansers selecting for resistant strains of bacteria?

3. Should farmers grow genetically engineered crops?

4. Is walking under a ladder bad luck?

5. Do cactus spines reduce herbivory?

6. How does the presence of predators influence the behavior of prey species?

How did you decide which questions could be answered scientifically?

After a good question is posed, scientists attempt to answer it by proposing possible explanations. These proposed explanations are called hypotheses. For some questions (see #5 above), the hypotheses can be a simple as “yes or “no”. Generally, it is more interesting if the question is phrased in such a way so that “yes” and “no” are not the only possible answers.

The Components of a Scientific Experiment

After coming up with several hypotheses, scientists design experiments to figure out which, if any, of their possible explanations might be right. Consider the following question and propose several hypotheses.

How do emissions from coal-powered power plants influence the productivity of agricultural fields?

Every experiment consists of a few basic parts:

Independent variable-

Dependent variable-

Standardized variables-

Levels of treatment-

Replication-

For the question posed above, identify possible variables (independent, dependent, and standardized), levels of treatment, method for replication, and predictions.

Summarizing Data

After coming up with your question, hypotheses, and predictions, scientists carefully design experiments and then collect, analyze, and interpret data. The key parts of an analysis are 1.) summarizing the data, and 2.) comparing the observed data with the data predicted for each of the hypotheses. Before you can perform any sort of comparison, you must first summarize your data by calculating descriptive statistics. There are two basic categories of descriptive statistics- Statistics of Central Tendency and Statistics of Dispersion. Both types of statistics are absolutely necessary to summarize a data set.

Statistics of Central Tendency

Mean, median, and mode are the three statistics of central tendency.

Mean= sum of the observed values (x) divided by the number of observations (n) = S x/n

Median= middle value in a data set

Mode= most common value for a data set

Example data set: 5, 8, 8, 8, 8, 9, 11, 12, 13, 27, 27

Mean= 136/11= 12.4 Median= 9 Mode= 8

Statistics of Dispersion

As mentioned above, there are two essential parts to summarizing a data set- statistics of central tendency AND statistics of dispersion. Consider the following two sets of test scores for two students:

Student #1: 74, 75, 76

Student #2: 50, 75, 100

For these two students, the means alone do not reflect the variation/consistency within the data sets. You would be hard pressed to say that the two students are equivalent students. In addition to the mean, you should report a statistic of dispersion- range, standard deviation, or variance.

PROCEDURE #1: Designing an Experiment

1. Working in groups of 4, come up with a question about how something might influence a person’s cardiovascular health.

2. How could you determine whether or not the factor you identified actually does influence cardiovascular health? Design an experiment. The first step is to figure out a way to measure cardiovascular health. Continue planning your experiment by identifying variables, levels of treatment, method for replication, and predictions.

PROCEDURE #2: Summarizing and Presenting Data

Suppose you’re comparing two samples of human subjects and their response to a diabetes drug. You may be interested in the specific effects of two different dosages of that drug on blood glucose levels (Sample A subjects receive a high dose, while Sample B subjects receive a lower dose). However, it is important that your two sample groups are standardized with respect to age and weight.

1. Summarize the data for Sample A ages and Sample B ages.

2. Summarize the data for Sample A weights and Sample B weights.

3. Present these summaries in graph form. Due in class on Monday, Sept. 11.

Sample A Ages / Sample A Weights / Sample B Ages / Sample B Weights
21 / 178 / 21 / 163
22 / 217 / 21 / 147
22 / 206 / 23 / 135
24 / 189 / 24 / 127
27 / 196 / 27 / 137
28 / 214 / 28 / 148
28 / 211 / 28 / 147
29 / 193 / 29 / 143
31 / 199 / 31 / 173
31 / 184 / 32 / 157
33 / 178 / 33 / 163
34 / 203 / 34 / 143

Calculating Summary Statistics with Excel

1. Enter your data in columns in Excel and move the cursor to an empty cell at the bottom of the first column (Sample A age)

2. To calculate mean of the numbers in the column, type =average(select the numbers)

3. Move to another empty cell below the column.

4. To calculate standard deviation of the same numbers, type =stdev(select the numbers)

Enter your summary statistics on a new data sheet:

Sample A
Age / Sample B
Age
Mean / 27.5 / 27.6
Standard Deviation / 4.4 / 4.5

Name______

Bio 120 Lab Homework #2

Due Monday, 9/11 at 9:00

1. Match the following terms to the correct definition. (3 pts)

_____standardized variable A. tentative explanation for an observation

_____level of treatment B. what an investigator varies during an experiment

_____dependent variable C. appropriate values to use for the independent variable

_____replication D. what an investigator measure, counts, or records during

an experiment

_____independent variable E. variable kept constant during the experiment

_____hypothesis F. number of times the experiment is repeated

2. Sample A and Sample B age graph with proper labels and units (3 pts)

3. Sample A and Sample B weight graph with proper labels and units (3 pts)

Graphs MUST be computer generated and stapled to this sheet.

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