All of the DOT worker characteristics are fully described in the 1991 Revised Handbook for Analyzing Jobs. The RHAJ is the foundation upon which the DOT was built.  Job Analysts used the RHAJ definitions to guide their coding of each of the observed worker characteristics.  

Number values used in Aptitudes, General Educational Development, and for Specific Vocational Preparation (SVP) represent a category describing a level of the functioning needed to learn/perform the occupation. Rather than being a percentile (%ile) value (which is a concept coming from the world of testing of individual performance relative to the performance of others), the number values represent a category describing an approximate level of the aptitude/functioning needed to learn/perform the occupation. The number 5 was used to mean that no aptitude in that area was needed for that occupation.  The original 1972 Handbook for Analyzing Jobs explains this on page 294.  Also see page 9-2 in the 1991 RHAJ chapter 9 for more detail.

• For Aptitudes [RHAJ Chapter 9], coding was done on a scale of 1-5 where 1 was high and 4 was low.  A rating of 5 meant that very little or no aptitude in this category was needed to learn/perform the occupation.
• For General Educational Development (GED) - [RHAJ Chapter 7], the scale ranged from 1-6 with 6 being high and one being low.  A value of 1 meant the lowest level, not inability.
• For Specific Vocational Preparation (SVP) - [RHAJ - Chapter 8], the scale ranged from 1-9 with 1 being low and 9 being very high.  
• For Temperaments (RHAJ Chapter 10), a rating of "present" was assigned if the job analyst judged that the worker would need to be adaptable to specific kinds of required job situations. 
• For Physical Demand and Environmental Conditions [RHAJ Chapter 12], ratings were based on a frequency scale of Not Present, Occasionally (up to 1/3 of the work day), Frequently (1/3-2/3 of the work day), and Constantly (> 2/3 of the work day).
• The Strength factor [RHAJ Chapter 12] is derived from a complex combination of factors including weights, forces, and worker positions.  

In its definition of each of these factors, the RHAJ presents a significant array of examples of work-related activity that best exemplifies each observed level. Each rating reflects the learning/performance requirements of a specific occupation.  It does NOT represent how this factor is distributed among the general population (as in a traditional bell-shaped curve model of frequency distribution).  

It is often very helpful to understand that these values represent requirements put upon the worker to learn/perform the occupation successfully. Absent a sufficient amount of a particular aptitude or other function, a worker is less likely to succeed at the occupation.

Because the ratings of occupations looked at occupations rather than people, the ratings are often skewed in surprising directions.  There are actually few occasions in which a "normal" bell-shaped distribution occurs.  It is helpful to be very aware of the frequency with which each value is observed.  The tables below show frequency counts for each of the factors across the DOT.  These counts are also neatly summarized in the electronic Pocket Guide to the DOT, which is also available in handy printed form at SkillTRAN.