The phrase "a first difference is that" is correct and usable in written English.
It can be used when comparing two or more items, highlighting the initial distinction between them.
Example: "When analyzing the two studies, a first difference is that the sample size in the first study was significantly larger than in the second."
Alternatives: "the first distinction is that" or "one key difference is that".
A second difference is that the recall period of both instruments is different: today for EQ-5D, versus the last four weeks (or one week in the acute version of the SF-36) for the SF-6D.
A second difference is that the Democratic Party is far less ideologically divided today than it was 50 years ago.
A second difference is that Hlinka's English is said to be rough, and it is not known if he speaks any French.
A third difference is that "Little Women" portrays a matriarchal household made up of a mother, her daughters and their cook.
A second difference is that, unlike most local authority-run pupil referral units, The Lighthouse Group takes teenagers before matters at school have escalated to the stage where they've been permanently excluded.
A third difference is that unlike 'way', dao may be used as a verb.
A second difference is that classical chaotic dynamics has a continuous energy spectrum associated with its motion.
A third difference is that this time there were kids fighting on both sides.
A second difference is that during the later period devaluations have invariably been effected by nations expanding their money supplies either by creating money to buy foreign currency, in the case of direct interventions, or by creating money to inject into their domestic economies, with quantitative easing.
A third difference is that the voxels selected on the basis of a semantic-factor-correlation-times-stability capture important but less stable representations distributed throughout the cortex, including frontal, parietal, and temporal areas that probably encode semantic information.
