People who are non-native speakers of English often misuse words or confuse pairs of similar words. You might be surprised to know that this is a very common problem for native speakers as well. Therefore, when you read articles written by native speakers of English, do not assume that all word usage is correct. Here is a short list of words and pairs of words that I often see misused by native and non-native speakers alike. More lists will follow in the weeks to come, so be sure to check back.

affect/effect:	These words are often confusing for both native and non-native speakers. Affect is a verb and effect is a noun. If you look in a dictionary you might find that effect can be used as a verb, but you should not follow this advice. The use of effect as a verb is old, awkward, and avoided by good writers.
case/patient:	A case is a particular instance. It can be evaluated, followed, and reported. A patient is a person who is under medical care. Do not call an animal a patient. Also a sick person who is not receiving treatment is not a patient.
follow/observe:	Serological tests commonly are used for the diagnosis of T. infections.
continually/continuously:	Continual means to happen over and over in frequent intermittent episodes. Continuous means to occur without interruption. Using continuous, for some reason, is very popular with Korean writers, usually in an incorrect manner. For example:

 The patient continually complained of chest pains for two weeks.

 he patient continuously complained of chest pains for two weeks.
(If this sentence is interpreted literally, then the patient did not eat or sleep. He did nothing but complain of chest pains. This might not be the best example to give you, but I think you can see how the two words should be applied in your own writing.)

dose/dosage:

A dose is the quantity to be administered at one time, or the total quantity administered. Dosage, which is the regulated administration of doses, is usually expressed in terms of a quantity per unit of time. Here is a correct example.

 The patient was given a dosage of 0.25 mg every four hours until the entire dose had been ingested.

negative/abnormal and positive/normal:	Negative and positive are words describing cultures, tests for microorganisms, tests for specific reactions, and reactions to tests. Normal and abnormal describe observations, results, or findings from examinations and tests.
principal/principle:	As a noun principal means a leader, and as an adjective it means the highest rank. A principle is a fundamental rule or law. The following example shows both words used correctly.

 The lack of money was the principal reason we could not continue our study of biological principles.

each other/one another

A dose is the quantity to be administered at one time, or the total quantity administered. Dosage, which is the regulated administration of doses, is usually expressed in terms of a quantity per unit of time. Here is a correct example.

 In this study we used varying amounts of........

 In this study we applied varying conditions to the.....

These sentences imply that the amounts and conditions were individually, continually, and perhaps randomly changed, whereas various would have implied that there was a selected set of amounts and/or changes that were used. In most studies, various would be the proper word for these sentences.

percent/percentage:	This is a very common mistake I see in papers that we receive. Percent should only be used when it is preceded by a number, e.g. ten percent). Use percentage in all other cases.
etc./and so on/and so forth:	First of all, let me say my personal feeling is that these words should be avoided in academic writing. They should certainly not be used several times throughout an article as I have often seen. Never use them with the phrase 'such as'. This phrase already indicates that your list is not complete and that you are only giving examples.
specially/especially	These two words are often confused, misused, and overused by Korean writers. The can sometimes mean the same thing, but specially never means 'more importantly' or 'moreover'. It usually means 'for a specific purpose or a designated reason'. Especially should rarely begin a sentence. Depending on the author's intent, phrases or words such as 'more importantly', 'moreover', 'notably', or 'specifically' are better choices. The following are two good examples of the words being used. The next two are bad examples of sentences I often see.

 The hotel was specially designed for wheelchair users.

 Drug X was effective in helping the patients lose weight, especially those over 35 years of age

.

 Drug X was effective in helping the patients lose weight. Specially, it reduced their cravings for food.

 (A better word choice here would be 'specifically'.)

 Plan A was chosen. Especially, it was cheaper, more effective, and easier to implement.

 (In this sentence, no adverb is necessary at the beginning. The information alone is sufficient).