Introduction: People who believe in astrology or horoscopes think that your birthdate determines your destiny, and that the relative positions of the planets can influence your daily life. The scientific community is in solid agreement that this is nonsense, but a large number of people still read the horoscopes in the newspaper in the hope that it will reveal some clues about their future. President Ronald Reagan even decided the dates of some important meetings after consulting with a "professional" astrologer! We will conduct an experiment that tests the validity of horoscopes. If astrology is valid, we should be able to repeatedly pick the ones that match our birthdays. The odds of selecting the right horoscope just by guessing are 1 in 12. If the results do not differ significantly from chance alone, then we must conclude that this horoscope system doesn't work. This is called the null hypothesis. The alternative hypothesis is that horoscopes do work more often than one would expect by chance alone. At the end of this study, we will be able to say which hypothesis is supported by the data.

Materials and Methods: Go to an astrology website and copy today's predictions for each of the signs, but remove the signs and randomize the predictions as coded letters A through L. Tomorrow, have students read the descriptions which correspond to each of the 12 signs of the zodiac. When done reading, ask students to choose the sign which best describes the day they had yesterday. Students only get one choice. Write choices in PEN (no second guessing) in the blank below. See the table below for information on the range of birthdates found under each sign of the zodiac.

For example, your predictions might look like this. Keep track of the actual predictions. In this example, the answers were:

Sign A: You will be where you are supposed to be. Rely upon intuition, timing. Keep the faith. Goal is closer than anticipated. Aquarian involved.

Sign B: Problems others can't solve will be regarded as easy for you. Scorpio people figure in unusual scenario. Moon position emphasizes success in finance, romance.

Sign C: Family differences arise in connection with payments, loans. Be familiar with accounting procedures, make intelligent concessions without watering down principles.

Sign D: Attention revolves around reputation, credibility, legal rights. Spotlight on proposals, partnership or marriage. You'll learn you are not alone. Cancer native plays major role.

Sign E: Fun, frolic behind the scenes. Lunar position emphasizes clandestine arrangements, hidden romance. Popularity increases as result of published material.

Sign F: People who said you were going off-track are proved wrong. New start was necessary. You are where you should be. Creative juices stir. Love not far away. Leo plays dynamic role.

Sign G: Contrary to family opinion, you are doing the right thing. Make travel arrangements. Your soul mate awaits. Focus on fitness, employment. Aries plays exciting role.

Sign H: Spiritual antennae up-you'll be guided in your actions. Define terms, ask for what you need and you shall receive. Taurus individual makes declaration of loyalty.

Sign I: Element of deception present, deliberate or otherwise. Fight for legal rights, insist on viewing results of accounting. Questions concerning inheritance arise.

Sign J: Good news. Sword of Damocles removed-bills will be paid, obligations met. You'll be aware of your own worth-which is plenty. Relative visits, compliments you on housekeeping.

Sign K: Lunar phase highlights promotion, production, recognition you should have received months ago. You'll get material reward.

Sign L: Keep plans flexible-individual who plans itinerary will be absent. Take charge of your own destiny-love relationship depends on ability to travel. Aries involved.

Results: Fill in the class results in the following table.

Analysis: There is a one in twelve (about 8%) probability of guessing correctly by chance alone, so in a class of 36 students we should expect 3 correct guesses even if horoscopes are meaningless. More than 3 in a class of 36 would suggest that horoscopes have some validity, and less than 3 of 36 would be even more difficult to explain! Be sure to use a large sample size (100 or more students should be sufficient). Small samples can over- or under-estimate the true pattern.

Questions: Consider and discuss the following questions.

1. Why was it important to conceal the identity of each of the signs of the zodiac from students while doing this study?

2. How often should we expect a false positive? Why do we need to know the expected number of false positives?

3. Did we get exactly the results predicted by the null hypothesis (~82% guessed wrong, ~8% guessed right)?

4. If the results are off from the expected, in which direction are they off? More than expected guessed wrong? More than expected guessed right?

5. Are they off by a little from the expected, or off by a lot? If the results are off by a lot, do we reject the null hypothesis? Have we considered sources of error?

6. Why would we have to agree if someone argued that we have not disproven astrology as a whole?

7. What does it mean when we say that a hypothesis must be testable and falsifiable? Why is this important?

8. Why is sample size important? Why can results vary from expected when sample sizes are small, even if there's nothing going on?

9. How could the alignment of the planets affect people based on their birthday? Is there any mechanism that makes sense? Does it seem likely that 1/12 of the world's population is having the same kind of day?

10. Do people who use horoscopes really believe in them, or are they just having fun? Discuss.