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Comma Splice

A comma splice results when a comma is mistakenly placed between two independent clauses that are not joined by a coordinating conjunction. (If the comma is omitted, the error is called a run-on sentence.) 

Comma splices and run-on sentences may be corrected in five principal ways:

  1. Use a period. Write two sentences. 
  2. Use a semicolon. 
  3. Use a comma and a coordinating conjunction (and, or, nor, but, for, so, and yet)
  4. Make one of the clauses dependent (subordinate), and use a subordinating conjunction such as after, although, because, before, if, since, though, unless, until, when, where, and while
  5. Reduce one of the independent clauses to a phrase or even a single word.
Student errors:
Environments are not static states, they are constantly changing and evolving.
Fourteen types of bacteria were used as colonists, their presence or absence was noted in ...
Mimicry has several distinct advantages for the mimic species, it does not itself have to spend extra energy on creating a chemical defense.
Environments are not static states; they are constantly changing and evolving.
Environments, constantly changing and evolving, are not static states.
Fourteen types of bacteria were used as colonists. Their presence or absence was noted in....
Fourteen types of bacteria were used as colonists, and their presence or absence was noted in....
Mimicry has several distinct advantages for the mimic species because it [the mimic species] does not have to spend....

Other common comma errors

A comma (from a Greek word meaning "to cut") indicates a relatively slight pause within a sentence. There are too many rules about commas to repeat here, so you may wish to check a writing resource. 
If after checking the rules you are still uncertain of whether or not to use a comma in a given sentence, read the sentence aloud and see if it sounds better with or without a pause, and then add or omit the comma. Do not just randomly scatter commas throughout your writing! 
A common student error is placing a comma between two verbs. Do not separate compound verbs with commas (correct: "Sam cycled and then swam.") Do separate full, independent clauses when joined by a coordinating conjunction (and, or, not, but, for, so, and yet. (correct: "I want to eat dinner, but I should work out first").
Student errors:
This last hypothesis, suggests to us that surpluses of food ....   (comma not needed)
The disease began in Sweden in 1972 and since then it has spread.   (comma needed)

Semicolon misuse 

Typographically, a semicolon is part comma, part period, and it does indeed function as a strong comma or as a weak period. It can never function as a colon. As a strong comma, it can be used to provide strong separation of two independent clauses with a coordinating conjunction (normally, a comma provides this separation) or to separate a series of phrases or clauses with internal commas. 
As a weak period, the semicolon joins independent statements that the writer wishes to bring together more closely than a period allows. Use a semicolon also before a conjunctive adverb (that is, a transitional word such as consequently, furthermore, however, moreover, nevertheless, therefore) connecting independent clauses, and put a comma after the conjunctive adverb. Students tend to overuse semicolons, which have specific, limited functions.
Student error:

Bell and Mitchell simulated ocean bacterial and algal growth using; marine algae, bacteria, ...

The algae appear to be healthy; however, the bacterial count continues to increase.

Sentence fragments 

A fragment is a part of a sentence set off as if it were a complete sentence. A sentence fragment is often an afterthought that really is an explanation or other addition to the previous sentence. With appropriate punctuation, a fragment can usually be connected to the previous sentence.
Student errors:
Although clutch sizes vary with feeding in other species of birds.
A possibility which is impossible for the first study.

Lack of parallel structure

Put parallel ideas and details in parallel constructions.
Student error: 
Algal filtrates were obtained using a centrifuge and sterile filtration.
Algal filtrates were obtained using centrifugation and sterile filtration.

Misplaced and dangling modifiers 

A modifier should appear close to the word it modifies (that is, describes or qualifies). If the modifier seems to modify the wrong word, it is called misplaced. Misplaced modifiers are often funny ("Orwell shot the elephant under pressure.") If the term it should modify appears nowhere in the sentence, the modifier is called dangling.
Student error:
After reading the original study, the article remains unconvincing. (Who read the study?) 

Nouns as modifiers

Avoid overuse of nouns as modifiers. We often use one noun to modify another, especially in the absence of an appropriate adjective form. Carefully conceived, such constructions can be both clear and concise ("child care center"). But overuse of nouns to modify other nouns can lead to awkward or confusing constructions, so keep two principles in mind. First, whenever possible, use a possessive or an adjective as a modifier. Second, use only short nouns as modifiers and use them only in sequences of no more than two or three words.
Student errors:
There was a steady increase in the bacteria count as the algae culture aged.
Foliar injury increases with simulated acid rainfall application duration.
There was a steady increase in the bacterial count as the algal culture aged.
Foliar injury increases with the length of time that simulated acid rainfall is applied.

Confused phrasing and word choice 

Say exactly what you mean. Read what you have written and, if appropriate, ask someone else to read it to ensure that your writing is correct and clear.
Student errors:
The only weakness in Juniper's study was a question I had regarding...
Reasons for the decline may be due to the loss of habitat space.
Three times a week the study would fly over the area to observe population estimates...
Overall their results do convene.

Erroneous word choice

The following pairs of words are often confused by students. Here, the examples demonstrate correct usage of the words.
affect - effect 
Usually affect is a verb, meaning "to influence," and effect is a noun, meaning "result." But effect occasionally is used as a verb meaning "to bring about."
Social activities may affect your grades, but the effect better be little, or we can effect a change in your life style!
farther - further 
Farther refers to additional distance, and further refers to additional time, amount, or other abstract matters.
You may be further from an "A" than you think, so when you study, go no farther than the best place to concentrate.
less - fewer 

Less refers to bulk amounts, while fewer refers to separate (countable) items.

Those with less knowledge receive fewer high grades.

between - among 
among Between is used when two are concerned (the word comes from "by twain" in Middle English), while among is used when more than two are concerned.
Between you and me, these mistakes are common among all of us.
feel - think
In common usage, feel means to sense, to be emotionally affected by something, or to have a general or thorough conviction of something. Think means to use reason or to examine with the intellect.
I think that you can write better than you have, while I feel encouraged by the improvements in your writing.
which - that
Use that in restricting (limiting) clauses: "The rocking chair that creaks is on the porch." You are distinguishing between two, or more, rocking chairs. 
Use which in nonrestrictive (in effect, parenthetic) clauses: "The rocking chair, which creaks, is on the porch." You have one rocking chair, and it creaks. 
The difference in these sentences is this: in the first, one rocking chair is singled out from several – the one that creaks; in the second, the fact that the rocking chair creaks is tossed in; it is not added for the purpose of identifying the one chair out of several. 
Note: Use who for people, in both restrictive and nonrestrictive clauses.
A technique that can improve your writing is proofreading, which can show you unintentional errors.
since - because
Since is often used to mean because: "Since you ask, I'll tell you." Its primary meaning, however, relates to time: "I've been waiting since noon." Most people now accept since in place of because; however, when since is ambiguous and may also refer to time ("Since he joined the navy, she found another boyfriend") it is better to say because or after, depending on which you mean.
Because you are intelligent and careful, your writing has improved since the beginning of this course.

by E.H. Williams, Biology Dept.

Parts have been taken directly from the following two sources (mostly from the first):

Barnet, S., & M. Stubbs. 1980. Practical guide to writing. Little, Brown and Co., Boston.
Fowler, H.R. 1983. The Little, Brown Handbook. Little, Brown and Co., Boston.


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