BFAS vs NEO

This article will briefly compare the IPIP items for the BFAS and NEO versions of the big five and explore the similarities and differences. The BFAS is, to a large degree, a streamlined version of the NEO, so this article will mostly take the form of clarifying which facets from the NEO are not represented in the BFAS. The following sentences all refer to the BFAS, unless otherwise stated.

Openness/Intellect

Imagination is mentioned briefly: "Do not daydream" (NEO), "Seldom daydream" (BFAS).
Artistic Interest is a critical component and is mentioned in 7/10 of the items for the "openness" aspect of "openness/intellect".
Emotionality has been totally removed from this trait but makes a brief appearance in the agreeableness trait instead: The same item, "feel others' emotions" features in the emotionality facet of NEO openness and in the Compassion aspect of BFAS agreeableness.
Adventurousness and Liberalism are absent.
NEO Intellect is virtually identical to BFAS Intellect except the BFAS aspect includes some references to speed of thought. NEO intellect seems more focussed on an interest in intellectual matters whereas BFAS Intellect seems like it would be more strongly correlated with IQ.

Summary: BFAS Openness/Intellect has trimmed away nearly every single reference to the NEO facets except for Artistic Interest and Intellect, which are the primary focus of the BFAS trait. Intellect probably correlates more strongly with IQ than anything in the NEO.

Conscientiousness

Summary: BFAS Industriousness is a mix of NEO Self-Efficacy and Self-Discipline. Orderliness is the same in both systems. BFAS Conscientiousness consists of 3 of the NEO facets (the most influential one being Orderliness) but Dutifulness, Achievement Striving and Cautiousness are absent.

Extraversion

Summary: Enthusiasm (BFAS) is a mix of Friendliness and Cheerfulness (NEO). Assertiveness is the same in both scales. Gregariousness, Activity Level and Excitement Seeking are absent.

Agreeableness

Trust is absent.
Morality is absent except for one item that is shared with "politeness": "Take advantage of others".
Altruism accounts for maybe 4/10 of the "compassion" items.
Cooperation accounts for maybe 4 of the Politeness items.
Modesty is largely absent except for the "Believe I am better than others" item which it shares with "politeness".
Sympathy (NEO) is similar to some of the items in "Compassion" (BFAS) but only directly shares one item: "Am not interested in other people's problems".

Summary: This one seems to touch on more of the NEO facets than some of the other traits. Compassion is a mix of Altruism and and sympathy, plus one item from NEO openness: "Feel others' emotions". Politeness mainly references cooperation but also touches on morality and modesty. Trust is the only facet that has been dropped entirely.

Neuroticism

Anxiety shares 2 items with Withdrawal: "Worry about things", "Am afraid of many things".
Volatility shares 4 items with Anger.
2 items from Withdrawal refer to depression.
Withdrawal and Self Consciousness share 1 item.
Immoderation is totally absent.
Withdrawal and Vulnerability share an item.

Summary: Volatility is basically just Anger but with a few additional items about general changes in mood. Withdrawal covers most of the remaining facets: Anxiety, Depression, Self Consciousness and Vulnerability. Immoderation is the only facet that is missing entirely.

How does the BFAS differ from the NEO, overall?

To a large extent the BFAS is a pared down version of the NEO. In some cases several facets have been squeezed into one "aspect" of the BFAS. In other cases an aspect is just equivalent to one facet. We can measure how significantly each trait has been altered by noting how many facets have been entirely removed from each:

3/6 facets are missing: Conscientiousness, Openness and Extraversion.
1/6 facets are missing: Agreeableness and Neuroticism.
Total missing facets: 11/30

Quick summary of missing Facets:
Openness: Emotionality (relocated to Agreeableness), Adventurousness and Liberalism.
Conscientiousness: Dutifulness, Achievement Striving and Cautiousness.
Extraversion: Gregariousness, Activity Level and Excitement Seeking.
Agreeableness: Trust.
Neuroticism: Immoderation.

Most Prominent NEO Facet in each BFAS trait:
Openness: Artistic Interests and Intellect.
Conscientiousness: Orderliness.
Extraversion: Assertiveness.
Agreeableness: Altruism or Cooperation.
Neuroticism: Anger.

Conscientiousness, Openness and Extraversion are significantly altered because the facets that would determine 50% of the trait score on a NEO test have been completely removed. Conscientiousness and Extraversion both turn 1 facet into an entire aspect. Orderliness and Assertiveness are largely unchanged and have gone from accounting for 17% to 50% of the total trait score. This is also largely true of Anger (NEO) and Volatility (BFAS).

Openness/Intellect is arguably the most changed because it relies almost entirely on just 2 facets (Imagination is barely mentioned) and adds the new element of speed and clarity of thought to the Intellect aspect.

Neuroticism is not heavily altered. Additional emphasis has been placed on anger. Immoderation has been dropped but otherwise this trait covers the same ground as NEO Neuroticism.

Agreeableness is the least changed trait because most of the facets are divided evenly between the 2 aspects and only Trust has been jettisoned.

Comparative Item Intercorrelations

The IPIP site contains tables that include data on how high or low the items are correlated in either system. This is basically a measure of the validity of each trait. 

Neuroticism: NEO: .22, BFAS: .22
Extraversion: NEO: .16, BFAS: .24
Openness: NEO: .17, BFAS: .22
Agreeableness: NEO: .13, BFAS: .22
Conscientiousness: NEO: .23, BFAS: .22
Overall average for each trait: NEO: .18, BFAS: .22

The intercorrelations for the BFAS trait items tend to be higher than those of the NEO. This does not mean that the BFAS is a better system. The BFAS is less comprehensive than the NEO. Of course it is possible to increase the correlation between items if you first reduce the total number of items. For example, a version of neuroticism that has just two items - Seldom feel blue, Often Feel Blue - Would have a correlation close to 1 because it is essentially just asking the same question twice in an inverted manner. Having a very high correlation number does not equate to an excellent measure of neuroticism because those 2 questions only tackle one component of neuroticism and ignore the other elements. The comparison of these two systems becomes a subjective question. Should trait items be extensive and comprehensive with a low degree of accuracy or should they be selective and exclusive with a high degree of accuracy? There is no definite right or wrong answer to this question.

Conclusion

Although a few things have been added (the IQ oriented elements of Intellect) and one or two things have been moved (Emotionality from Openness to Agreeableness) the main differences are in the paring down of the total number of facets (roughly 1/3 are gone) and the shift of emphasis across the facets (some facets account for 50% of a trait score (e.g. Orderliness) while others only account for 5% (e.g. Imagination)). The BFAS is a simplified, less comprehensive, more focussed version of the NEO, which captures a less expansive but more internally consistent representation of an individual's personality.

Sources:
BFAS Scoring Keys
NEO Facets Key (ori.org)
https://ipip.ori.org/newNEO_FacetsTable.htm
https://ipip.ori.org/BFASTable.htm