Which of the following is true regarding DMFT index?
The Decayed - Missing -Filled Teeth Index (DMFT Index) was developed by Henry T. Klein, Carrole E. Palmer and Knutson J. W. in  1938 to determine the prevalence of coronal caries.
Principles and rules in recording DMFT: 

No tooth must be counted more than once. It is either decayed, missing, filled or sound.
Decayed, missing, and filled teeth should be recorded separately since the components of DMF are of great interest. 
When counting the number of decayed teeth, also include those teeth, which have restorations with recurrent decay. 
Care must be taken to list as missing only those teeth, which have been lost due to decay. Also included should be those teeth which are so badly decayed that they are indicated for extraction. The following should not be counted as missing A) Unerupted teeth B) Missing teeth due to accident  C) Congenitally missing teeth D) Teeth that have been extracted for orthodontic reasons. 
A tooth may have several restorations but it is counted as one tooth. 
Deciduous teeth are not included in DMF count.
Tooth is considered to be erupted when the occlusal surface or incisal edge is totally exposed or can be exposed by gently reflecting the overlying gingival tissue with the mirror or explorer. 
A tooth is considered to be present even though the crown has been destroyed and only the roots are left.
Teeth extracted for orthodontic treatment are considered in missing.
Primary teeth are included in the scoring.
A tooth is considered to be present if the crown is destroyed and only roots are present.
Teeth with secondary caries comes under filled teeth.