The Simplified Oral Hygiene Index (OHIS) differs from the original OHI (The Oral Hygiene Index) in the number of the tooth surfaces scored (6 rather than 12), the method of selecting the surfaces to be scored, and the scores, which can be obtained. The criteria used for assigning scores to the tooth surfaces are the same as those use for the OHI (The Oral Hygiene Index).
The OHIS, like the OHI, has two components, the Debris Index and the Calculus Index. Each of these indexes, in turn, is based on numerical determinations representing the amount of debris or calculus found on the preselected tooth surfaces.
SELECTION OF TOOTH SURFACES
The six surfaces examined for the OHIS are selected from four posterior and two anterior teeth.
In the posterior portion of the dentition, the first fully erupted tooth distal to the second bicuspid (15), usually the first molar (16) but sometimes the second (17) or third molar (18), is examined. The buccal surfaces of the selected upper molars and the lingual surfaces of the selected lower molars are inspected.
 In the anterior portion of the mouth, the labial surfaces of the upper right (11) and the lower left central incisors (31) are scored. In the absence of either of this anterior teeth, the central incisor (21 or 41 respectively) on the opposite side of the midline is substitted.
Criteria for classifying debris
Scores 
Criteria 
0 
No debris or stain present 
1 
Soft debris covering not more than one third of the tooth surface, or presence of extrinsic stains without other debris regardless of surface area covered 
2 
Soft debris covering more than one third, but not more than two thirds, of the exposed tooth surface. 
3 
Soft debris covering more than two thirds of the exposed tooth surface. 
Criteria for classifying calculus
Scores 
Criteria 
0 
No calculus present 
1 
Supragingival calculus covering not more than third of the exposed tooth surface. 
2 
Supragingival calculus covering more than one third but not more than two thirds of the exposed tooth surface or the presence of individual flecks of subgingival calculus around the cervical portion of the tooth or both. 
3 
Supragingival calculus covering more than two third of the exposed tooth surface or a continuos heavy band of subgingival calculus around the cervical portion of the tooth or both. 
After the scores for debris and calculus are recorded, the Index values are calculated. For each individual, the debris scores are totaled and divided by the number of surfaces scored. At least two of the six possible surfaces must have been examined for an individual score to be calculated. After score for a group of individual is obtained by computing the average of the individual scores. The average individual or group score is known as the Simplified Debris Index (DIS).
The same methods are used to obtain the calculus scores or the Simplified Calculus Index (CIS).
The average individual or group debris and calculus scores are combined to obtain the Simplified Oral Hygiene Index.
The CIS and DIS values may range from 0 to 3; the OHIS values from 0 to 6. These values are just half the score magnitude possible with the OHI (Oral Hygiene Index).
The following example shows how to calculate the index. The scores for debris and calculus should be tabulated separately and index for each calculated independently, but in the same manner.
Debris

Right molar 
Anterior 
Left molar 
Total 

Buccal 
Lingual 
Labial 
Labial 
Buccal 
Lingual 
Buccal 
Lingual 
Upper 
3 
 
2 
 
3 
 
8 
 
Lower 
 
2 
 
1 
 
2 
1 
4 
Debris Index = (The buccalscores) + (The lingualscores) / (Total number of examined buccal and lingual surfaces).
Debris Index = (9+4) / 6 = 2.2 
Calculus

Right molar 
Anterior 
Left molar 
Total 

Buccal 
Lingual 
Labial 
Labial 
Buccal 
Lingual 
Buccal 
Lingual 
Upper 
1 
 
0 
 
1 
 
2 
 
Lower 
 
1 
 
2 
 
2 
2 
3 
Calculus Index = (The buccalscores) + (The lingualscores) / (Total number of examined buccal and lingual surfaces).
Calculus Index = (4+3) / 6= 1.2 
The average individual or group debris and calculus scores are combined to obtain simplified Oral Hygiene Index, as follows.
Oral Hygiene Index 
= 
Debris Index + Calculus Index 
2.2 + 1.2 = 3.4 