Common punctuation errors at SWS

Comma splice masked by a transitional expression.

Never join two sentences with a comma followed by a transitional expression. I regularly see scientific writing (often by non-native speakers) in which two sentences are joined with a comma followed by a conjunctive adverb or other transitional expression, resulting in a comma splice. The most common error of this type involves joining two sentences with the word however:

The algorithm is correct, however it is not efficient.

Although I see this usage often, and the meaning is generally not obscured, every English usage book I've checked states that this usage is incorrect, and that conjunctive adverbs such as 'however' should not be used to link two sentences joined by a comma; use a period or a semi-colon instead. For example, the above sentence could be rewritten as follows:

The algorithm is correct; however it is not efficient.
The algorithm is correct. However, it is not efficient.

Alternatively, you can replace the conjunctive adverb with a coordinating conjunction such as 'and' or 'but':

The algorithm is correct, but it is not efficient.

Another option is to remove the conjunctive adverb and instead use a subordinating conjunction, such as 'although':

The algorithm is correct, although it is not efficient.
Although the algorithm is correct, is it not efficient.
The algorithm is not efficient, although it is correct.

Learn to distinguish conjunctive adverbs from subordinating and coordinating conjunctions; they do not function in the same way. More examples are given below.

Cooordinating and subordinating conjunctions may be preceded by a comma when they link independent clauses. Conjunctive adverbs and other transitional phrases cannot be preceded by a comma when they link independent clauses.

Examples of correct punctuation: Examples of incorrect punctuation:

The code is correct, but it is poorly written and terribly inefficient. The code is correct, however it is poorly written and terribly efficient.

American science is currently underfunded, so most students now prefer to work in Europe. American science is currently underfunded, as a result most students now prefer to work in Europe.

Her best ideas usually occur when she's relaxed, though her last breakthrough came only one hour before the deadline. Her best ideas usually occur when she's under pressure, for example, her last breakthrough came only one hour before the deadline.

We worked on it all day but the code failed to compile, so we gave up and went to dinner. We worked on it all day but the code failed to compile, therefore we gave up and went to dinner.

There have been many recent breakthroughs in this area, although the most up-to-date methods are not yet applied in practice. There have been many recent breakthroughs in this area, nevertheless the most up-to-date methods are not yet applied in practice.

The coordinating conjunctions: and, but, or, nor, for, so, and yet. Some conjunctive adverbs: accordingly, also, anyway, besides, certainly, conversely, finally, furthermore, hence, however, indeed, instead, likewise, meanwhile, moreover, nevertheless, next, nonetheless, now, otherwise, similarly, specifically, still, subsequently, then, therefore, thus

Some subordinating conjunctions: after, although, as, as long as, because, before, even if, even though, if, now, provided, though, unless, until, when, while Some transitional phrases: after all, as a matter of fact, as a result, at any rate, at the same time, even so, for example, for instance, in addition, in conclusion, in fact, in other words, in the first place, on the contrary, on the other hand

Cooordinating and subordinating conjunctions may be preceded by a comma when they link independent clauses.	Conjunctive adverbs and other transitional phrases cannot be preceded by a comma when they link independent clauses.
Examples of correct punctuation:	Examples of incorrect punctuation:
The code is correct, but it is poorly written and terribly inefficient.	The code is correct, however it is poorly written and terribly efficient.
American science is currently underfunded, so most students now prefer to work in Europe.	American science is currently underfunded, as a result most students now prefer to work in Europe.
Her best ideas usually occur when she's relaxed, though her last breakthrough came only one hour before the deadline.	Her best ideas usually occur when she's under pressure, for example, her last breakthrough came only one hour before the deadline.
We worked on it all day but the code failed to compile, so we gave up and went to dinner.	We worked on it all day but the code failed to compile, therefore we gave up and went to dinner.
There have been many recent breakthroughs in this area, although the most up-to-date methods are not yet applied in practice.	There have been many recent breakthroughs in this area, nevertheless the most up-to-date methods are not yet applied in practice.
The coordinating conjunctions: and, but, or, nor, for, so, and yet.	Some conjunctive adverbs: accordingly, also, anyway, besides, certainly, conversely, finally, furthermore, hence, however, indeed, instead, likewise, meanwhile, moreover, nevertheless, next, nonetheless, now, otherwise, similarly, specifically, still, subsequently, then, therefore, thus
Some subordinating conjunctions: after, although, as, as long as, because, before, even if, even though, if, now, provided, though, unless, until, when, while	Some transitional phrases: after all, as a matter of fact, as a result, at any rate, at the same time, even so, for example, for instance, in addition, in conclusion, in fact, in other words, in the first place, on the contrary, on the other hand