it utilizes a 4-point format in which respondents are asked to rate items pertaining to their problems . the self - report screening form ( caars - self report : screening version , caars - s : sv ) which was used in this study , has 30 items that assess adhd symptoms according to the 4 edition of diagnostic and statistical manual of mental disorders ( dsm - iv ) ( 3 ) . the subscales derived from the questionnaire are : 
 inattentive ( 9 items),hyperactive - impulsive ( 9 items),total adhd symptoms , adhd index ( 12 items ) . inattentive ( 9 items ) , hyperactive - impulsive ( 9 items ) , adhd index ( 12 items ) . we prepared and used the computerized version of the questionnaire to benefit from rapid scoring , increased accuracy , easy distribution of the questionnaire to the participants by e - mail , and making a database for statistical analysis more easily . a total number of 530 questionnaires were returned among which 130 participants answers were incomplete or scratched , and 400 questionnaires were filled out completely . this indicates the importance and need of proper estimation of missed or scratched answers in the questionnaire . , it should be taken into account that the study was performed on a population of university students . the participants distribution of age , gender and education we performed all statistical tests and calculations using the  matlab 2009a  software . the lilliefors test is a 2-sided goodness - of - fit test suitable when a fully - specified null distribution is unknown and its parameters must be estimated , i.e. it is a kolmogorov - smirnov test with unknown null distribution . the statistical distribution of the subscales is shown in figure 2 . the raw scores of the caars are converted to t - scores to eliminate the gender and age effects . the t - scores used in caars are linear ones which do not change the actual distributions of the variables ( 1 ) ; therefore , the variables which are not normally distributed in the raw data will continue to be distributed in this way after being converted to t - scores . ( caars : conners adult adhd rating scale ) to investigate the validity of the estimated missed or scratched answers , we assumed each question is missed individually , and estimated its answer using the average of other answers in its group . then , we compared the original answers with their calculated estimations for each question to find any significant difference . we considered the average of the other 8 questions as the estimation of the answer to the first question and evaluated if there is a significant difference between this estimation and real answers to the first question . to investigate the veracity of this approach , we used kruskal - wallis test which is a non - parametric version of anova . in the next step , we used the multiple comparison procedure ( mcp ) to evaluate the significance of the differences between the original and estimated answers in each group . since the chance of incorrectly finding a significant difference would increase with the number of comparisons , using the mcp can provide an upper bound on the probability that any comparison could be incorrectly found significant . in some questionnaires , the components ( questions ) are highly correlated ( redundant ) . in such cases , an effective procedure for performing this operation is principal components analysis , pca ( 10 ) . pca involves transforming a number of possibly correlated variables into a smaller number of uncorrelated variables called principal components . this technique which is closely related to factor analysis is used for extracting principal components of the caars questionnaire . it utilizes a 4-point format in which respondents are asked to rate items pertaining to their problems . the self - report screening form ( caars - self report : screening version , caars - s : sv ) which was used in this study , has 30 items that assess adhd symptoms according to the 4 edition of diagnostic and statistical manual of mental disorders ( dsm - iv ) ( 3 ) . the subscales derived from the questionnaire are : 
 inattentive ( 9 items),hyperactive - impulsive ( 9 items),total adhd symptoms , adhd index ( 12 items ) . inattentive ( 9 items ) , hyperactive - impulsive ( 9 items ) , adhd index ( 12 items ) . we prepared and used the computerized version of the questionnaire to benefit from rapid scoring , increased accuracy , easy distribution of the questionnaire to the participants by e - mail , and making a database for statistical analysis more easily . a total number of 530 questionnaires were returned among which 130 participants answers were incomplete or scratched , and 400 questionnaires were filled out completely . this indicates the importance and need of proper estimation of missed or scratched answers in the questionnaire . , it should be taken into account that the study was performed on a population of university students . the lilliefors test is a 2-sided goodness - of - fit test suitable when a fully - specified null distribution is unknown and its parameters must be estimated , i.e. it is a kolmogorov - smirnov test with unknown null distribution . the statistical distribution of the subscales is shown in figure 2 . the raw scores of the caars are converted to t - scores to eliminate the gender and age effects . the t - scores used in caars are linear ones which do not change the actual distributions of the variables ( 1 ) ; therefore , the variables which are not normally distributed in the raw data will continue to be distributed in this way after being converted to t - scores . to investigate the validity of the estimated missed or scratched answers , we assumed each question is missed individually , and estimated its answer using the average of other answers in its group . then , we compared the original answers with their calculated estimations for each question to find any significant difference . we considered the average of the other 8 questions as the estimation of the answer to the first question and evaluated if there is a significant difference between this estimation and real answers to the first question . to investigate the veracity of this approach , we used kruskal - wallis test which is a non - parametric version of anova . in the next step , we used the multiple comparison procedure ( mcp ) to evaluate the significance of the differences between the original and estimated answers in each group . since the chance of incorrectly finding a significant difference would increase with the number of comparisons , using the mcp can provide an upper bound on the probability that any comparison could be incorrectly found significant . in some questionnaires , the components ( questions ) are highly correlated ( redundant ) . in such cases , an effective procedure for performing this operation is principal components analysis , pca ( 10 ) . pca involves transforming a number of possibly correlated variables into a smaller number of uncorrelated variables called principal components . this technique which is closely related to factor analysis is used for extracting principal components of the caars questionnaire . the characteristics of the participants in age , gender and education are demonstrated in figure 1 . it shows that their distribution and consequently the distribution of raw scores is not normal ; thus , non - parametric tests are used for statistical evaluations . this table shows each question belongs to which group ( 9 in inattention , a ; 9 in impulsivity , b ; 18 in adhd , c ; and 12 in adhd index , d ) . p - values less than 0.01 indicate significant difference between original answer and its estimation . results indicate that a significant difference ( df = 399 , p<0.01 ) exists between the original and estimated answers for some questions ; therefore , the substitution method is not appropriate for their estimation . the results of kruskal - wallis test for comparison between original and estimated answers in caars questionnaire p - values less than 0.01 are considered to have significant difference . as table 1 demonstrates , this estimation is significantly different from the original value for questions 13 , 20 , 28 , 29 ( inattentive subscale ) , 4 , 15 , 26 ( hyperactive/ impulsive subscale ) , 4 , 13 , 15 , 18 , 20 , 26 , 28 , 29 ( adhd subscale ) and for 3 , 7 , 10 , 14 , 16 , 22 , 23 , 27 , 30 ( adhd index ) . the results of mcp shows that the estimation , when evaluated in the whole group , is not significantly different ( =0.001 ) from the original value in each subscale . figure 3 indicates the results of pca which transforms a number of possibly correlated variables into a smaller number of uncorrelated variables . the eigen values for all components drived from caars questionnaire based on principal component analysis ( pca ) . the distribution of t - scores in the statistical population for each subscale is demonstrated in figure 4 . the t - scores used with caars have a mean of 50 and a standard deviation of 10 . values around 50 indicate that the participant is in the average range in that subscale whereas higher t - scores represent a problem ; lower t - scores suggest that the participant does not present particular symptoms . the t - scores distribution diagram of participants in four subscales of caars ( inattention , hyperactivity , adhd and adhd index ) . t - scores distribution in the statistical population indicates that in the  inattention  group ( a ) , 44% of the participants were in the average range ( regarding their age and gender ) ; 10 % were very much above the average having high levels of symptoms that could meet the diagnostic criteria for attention - deficit / hyperactivity disorder ; and 8% were much below the average . in the  impulsivity  group ( b ) , the average range contains 38 % of the participants . 6 % were very much above the average ; and 11% were much below the average . in the  adhd group ( c ) , 43% of the statistical population was in the average range ; 13% was very much above the average ; and 7% was quite below the average . in the  adhd index group ( d ) , 56% of the statistical population was in the average range ; only 1% were very much above the average ; and 6% much below the average . adhd is a common problem in adults which can be evaluated using some self - report questionnaires including the conners adult adhd rating scale ( caars ) . a main problem in self - report measures is the high probability of missing some questions by adhd patients which can interfere with scoring of the questionnaire . in this case , to score the questionnaire , the operator should complete the missed answer(s ) . in this study , we evaluated the estimation of the missed answers in the self - report screening version of caars . the kruskal - wallis results revealed that the differences between original and estimated values were significant for some particular questions . four of these questions are in the  inattention  subscale , 3 in  hyperactive / impulsive  , 8 in  adhd  and 9 in  adhd index  . it is noteworthy that the estimation of a particular question in a or b subscale is obtained by averaging 8 other questions in that subscale while its estimation in c subscale is obtained by averaging 17 other questions . individual evaluation of each question shows that only the estimation of the answers in 11 questions can be valid . therefore , when a high percentage of the participants in one study have missed a particular question , it should be noted that the estimation should not be used for the critical questions mentioned in table 1 and the incomplete questionnaires should be removed from the study . however , the results of mcp , when evaluated in the whole groups , indicate that the estimations did not differ significantly from their original values in four subscales , i.e. the result of the whole groups will not change with this estimation . eight principal components were extracted by means of pca which transforms a number of possibly correlated variables into a smaller number of uncorrelated variables called principal components . the first component accounts for as much of the variability in the data as possible , and each succeeding component accounts for as much of the remaining variability as possible . the novelty of this study is in the evaluation method of estimation and taking into account the iranian population . regarding semi - similar studies , pca is used as a factor analysis method in translated versions of adhd questionnaires ( 11 - 13 ) . in the french version of asrs , self - report adult questionnaire is considered and factor analyses retrieved the original two - factor structure although impulsivity items clearly load on a specific third factor in students only(11 ) . also german and chinese children are respectively investigated ( 12,13 ) . psychometric properties of caars and fbb - hks questionnaires are compared for the former and both instruments scores showed reliability as well as factorial and convergent / discriminant validity ( 12 ) . for the latter , mixed findings with respect to the psychometric properties of the chinese translation of the sdq are reported ( 13 ) . considering adult iranian population , one study have reported the prevalence of 3.7% for adhd in a group of 244 university students , but no estimation or validity evaluation were investigated ( 14 ) . in summary , although the kruskal - wallis results revealed that the difference between the original and estimated values is significant for some particular questions individually , the results of mcp , when evaluated in the whole group , indicate that the estimations did not differ significantly from their original values in any of the four subscales , i.e. the result of the whole group will not change with this estimation . the obtained results can emphasize the fact that this questionnaire is mainly designed for screening purposes , and this estimation does not change the results of groups when a question is missed randomly . nevertheless , if many of the participants missed a critical question , this substitution would not be valid and more considerations should be paid in such cases .
objectiveconners adult adhd rating scale ( caars ) is among the valid questionnaires for evaluating attention - deficit / hyperactivity disorder in adults . 
 the aim of this paper is to evaluate the validity of the estimation of missed answers in scoring the screening version of the conners questionnaire , and to extract its principal components.methodthis study was performed on 400 participants . 
 answer estimation was calculated for each question ( assuming the answer was missed ) , and then a kruskal - wallis test was performed to evaluate the difference between the original answer and its estimation . in the next step , 
 principal components of the questionnaire were extracted by means of principal component analysis ( pca ) . 
 finally the evaluation of differences in the whole groups was provided using the multiple comparison procedure ( mcp).resultsfindings indicated that a significant difference existed between the original and estimated answers for some particular questions . 
 however , the results of mcp showed that this estimation , when evaluated in the whole group , did not show a significant difference with the original value in neither of the questionnaire subscales . 
 the results of pca revealed that there are eight principal components in the caars questionnaire.conclusionthe obtained results can emphasize the fact that this questionnaire is mainly designed for screening purposes , and this estimation does not change the results of groups when a question is missed randomly . notwithstanding 
 this finding , more considerations should be paid when the missed question is a critical one .