A dichotomous key is a classification tool that is based on opposing statements, usually on the presence or absence of certain physical characteristics. By identifying a whole series of differences, it is possible to narrow the field until a particular specimen is correctly identified. Dichotomous keys are frequently used in science, for example in biology or geology. To create a dichotomous key, select the characteristics you can use to differentiate the specimens and then make them in the form of increasingly specific statements or questions to narrow the field.
Steps
Part 1 of 3: Analyzing the Specimens
Step 1. List the features
Start by considering the specimens you intend to identify and place in a dichotomous key. Identify the characteristics that define the elements you are looking at and start listing them.
- If you want to create a dichotomous key to a series of animals, you might consider characteristics such as: they have feathers; they swim; they have legs; etc.
- For example, if you are trying to differentiate between big cats, you may notice that some are brown, others black; some have striped coats, others spotted; some have a long tail, some have a short tail, and so on.
Step 2. Look for principles of exclusion
The dichotomous keys work through a process of elimination; it is therefore necessary to identify characteristics that can serve to differentiate the specimens you are observing. For example, if some have feathers while others have fur, then "having feathers" is a good distinguishing feature.
A characteristic shared by all animals, on the other hand, is not a good distinctive element. For example, since all big cats are warm-blooded, it would be pointless to use this feature in the dichotomous key
Step 3. Define the more general characteristics
You have to create a dichotomous key based on increasingly narrow differentiations, so you will have to sort the characteristics of the specimens from the most general to the most specific. This will serve to divide the specimens into smaller and smaller groups. For instance:
- If you are making a dichotomous key to big cats, you may notice that some of the felines you are analyzing have dark fur, while others have light fur; that all are short-haired; that some have a long tail, while others don't have one at all.
- You could start with a question or statement about the color of the coat. It would be useless to make one on the length of the hair, since they all have it short. At that point you can continue with a question about the length of the tail, since the tail is not a common element for all cats and therefore constitutes a less general characteristic.
Part 2 of 3: Creating the Dichotomous Key
Step 1. Formulate a series of differentiation steps
You can choose whether to use questions or affirmations, although questions may be the most intuitive method. Either way, each question or statement should divide the specimens you are looking at into two groups.
- For example, "The feline has a solid-colored coat" or "The feline has a coat with a pattern" are statements you can use to divide the specimens into two groups.
- Or you can ask the question: "Does the feline have a solid color coat?". If the answer is "Yes", then the feline belongs to the group that has a solid color coat; if the answer is "No", then the feline belongs to the group that has the coat with a design.
Step 2. Divide the specimens into two groups
This is the first step of differentiation and should be based on the more general aspects of the specimens, so identify those aspects in the list of physical characteristics that you have compiled. You can indicate the two groups with the letters A and B.
- For example, you can separate felines based on the type of coat, whether it is a single color or has a pattern.
- Similarly, if you notice that all the specimens either have feathers or have scales, these could be groups A and B. You can start creating the key with the question: "Does the animal have feathers?".
Step 3. Divide each of the two groups into two subgroups
Group A and group B should be divided into two further groups (C and D) on the basis of more specific distinctive characteristics.
- For example, you might notice that some of the animals in group A swim while others don't. This differentiation can constitute the subgroups C and D of group A.
- Similarly, you may notice that some of the animals in Group B have legs while others do not. This differentiation can constitute the subgroups C and D of group B.
Step 4. Continue forming subgroups
Ask more and more specific questions or statements based on the physical characteristics you have identified. Find characteristics that can divide the specimens into groups E / F, G / H, and so on. Eventually you will come up with questions that will make you differentiate only two specimens; at that point the dichotomous key will be complete.
- As you progress through the contrasting features, some specimens will be differentiated before you get to the end of the key. For example, if you are observing birds and reptiles, you will first need to divide them into their respective groups, then divide the birds.
- Two of the birds are swimming, one is not; the land bird will be identified as such, but you will need to further differentiate the aquatic ones.
- Let's say that one of the swimming birds belongs to a marine species and the other is not; this will allow you to identify specimens more precisely (for example, a seagull and a duck).
Part 3 of 3: Complete the Dichotomous Key
Step 1. Make a pattern
The dichotomous key can be text-only and simply consist of a series of questions; however, organizing the material in graphic form can help. For example, you can create a "tree diagram", where each new branch represents a level of differentiation.
You can also try to organize the dichotomous key in a flowchart. For example, enter a question like "Does the feline have a dark coat?" and start from the square two arrows, one for the "Yes" and one for the "No", which go in different directions; the arrow can lead to another box where you ask the next question
Step 2. Test the dichotomous key
Once you have completed the key with all the necessary information, follow it with a copy in mind to see if it works. For example, let's say you have created a dichotomous key that serves to identify various animals; choose a specimen and follow the questions until you identify it through the elimination process:
- Question: "Does the animal have feathers?" Answer: "No" (it has scales, so it's a reptile).
- Question: "Does the reptile have legs?" Answer: "No" (it's a snake - a cobra or a python, depending on the species considered).
- Question: "Does the snake have a hood?" Answer: "No" (so it's not a cobra).
- The specimen is identified as a python.
Step 3. Resolve any problems
The key may be malfunctioning and you need to fix it. For example, you may not have sorted the questions according to progressive specificity and therefore need to reorganize them. Or the key may not break up the specimens in the most logical way and you need to rephrase the questions.
- For example, "Does the feline have a solid or striped coat?" it is not a useful question in a dichotomous way. It can serve to distinguish single-colored and striped felines from spotted ones, but since the solid-colored coat and the striped coat are themselves very different, it is not a useful category to work with.
- Instead, you should first ask yourself if the coat is a single color or has a design and then move on to the next level with questions like, "Does the feline have a black color coat?" and "Does the feline have a striped coat?".
