Bookmarking Auto on Info pages Supporting Auto on Info Home or Site Layout or Site Map or Site Index or Quick Jumps or Search Engine Directory or The MartAuto InformationReliability Information or Reliability Index Values or Reliability Percentranks or Reliability Grades - High Standards or Reliability Grades - Exacting Standards or Reliability ScoresMethodology

Reliability Index Value and Average To generate the reliability index value for a motor vehicle model, the 1996-1998 Buying Guides by the Consumers Union and the April 1998, the April 1999, the April 2000, the April 2001, the April 2002, the April 2003, the April 2004, the April 2005, and the April 2006 issues of Consumer Reports were used. In the “Reliability Records” portion of the 1996 Buying Guide appear charts for many of the motor vehicle models sold in the U.S. In each model chart, each of 16 trouble spots is rated for each of eight years, from 1987 through 1994. To compute an index value for a motor vehicle model manufactured in the years of 1987-1988, the following was done: 1.      The trouble spots of “Manual Transmission,” “Clutch,” “Brakes” and “Exhaust” were eliminated. The first two were eliminated as most vehicles of any description come with automatic transmission, by far the more popular choice. The latter two were eliminated as it seemed nearly impossible to imagine that brake work today is anything other than replacement of brake linings/pads and exhaust work is simply replacement of muffler and/or exhaust pipes, something that now carries a lifetime warranty, although the author’s imagination may be somewhat limited by his rather exclusive use of Toyota motor vehicles for the past nearly 18 years. 2.   If any of Consumer Reports ratings for the remaining 12 trouble spots were missing (due to insufficient data) for either of the two years, the model received no index value. Otherwise, the trouble spots in the chart that earned Consumer Reports’ lowest rating (highest frequency of reported car problems) in either of the two model years were counted and this count was multiplied by –1. Then the trouble spots in the chart that earned Consumer Reports’ second lowest rating (second highest frequency of reported car problems) in either of the two model years were counted and this count was multiplied by – ½. Then the trouble spots that earned Consumer Reports’ highest rating (lowest frequency of reported car problems) in either of the two model years were counted and this count was multiplied by +1. Then the trouble spots that earned Consumer Reports’ second highest rating (second lowest frequency of reported car problems) in either of the two model years were counted and this count was multiplied by + ½. 3.   Finally, the four numbers that were obtained in 2 were added together and divided by 24. This is the model’s Reliability Index Value for the pair of manufacturing years 1987-1988. So, the index value for a model manufactured in the years of 1987-1988 may vary from +1, the highest value, to –1, the lowest value.

 

Reliability Index Value and Average continued Similarly, Reliability Index Values for motor vehicle models manufactured in the years of 1988-1989 were obtained from the 1997 Buying Guide, Reliability Index Values for motor vehicle models manufactured in 1989-1990 were obtained from the 1998 Buying Guide, Reliability Index Values for motor vehicle models manufactured in the years 1990-1991 were obtained from the April 1998 issue of Consumer Reports, and Reliability Index Values for motor vehicle models manufactured in the years 1991-1992 were obtained from the April 1999 issue of Consumer Reports. The Reliability Index Values for motor vehicle models manufactured in the years 1992-1993 were obtained from the April 2000 issue of Consumer Reports in a similar manner as well; however, as Consumer Reports had by its April 2000 issue eliminated the trouble spots of “Manual Transmission” and “Clutch,” it was unnecessary to do so as part of the above step 1. Similarly, the Reliability Index Values for motor vehicle models manufactured in the years 1993-1994 were obtained from the April 2001 issue of Consumer Reports, the Reliability Index Values for motor vehicle models manufactured in the years 1994-1995 were obtained from the April 2002 issue of Consumer Reports, the Reliability Index Values for motor vehicle models manufactured in the years 1995-1996 were obtained from the April 2003 issue of Consumer Reports, the Reliability Index Values for motor vehicle models manufactured in the years 1996-1997 were obtained from the April 2004 issue of Consumer Reports, the Reliability Index Values for motor vehicle models manufactured in the years 1997-1998 were obtained from the April 2005 issue of Consumer Reports, and the Reliability Index Values for motor vehicle models manufactured in the years 1998-1999 were obtained from the April 2006 issue of Consumer Reports. It must be noted that beginning with the April 2002 issue, the paint/trim and rust categories were merged into the paint/trim/rust category and a new category for power equipment was added. Hence, the reliability index values and averages for 1994-1995, 1995-1996, 1996-1997, and 1997-1998 may not be entirely comparable with those from previous years. It also must be noted that beginning with the April 2006 issue, a new category (drive system) was added. Hence, the definition of the reliability index value shifted a tad and the divisor (24) mentioned in paragraph 3 above became 26. Note that from the method of computation and Consumer Reports timing of its surveys, the Reliability Index Values reflect a model’s performance for a vehicular age range of 6 to 8 years. This was done with the intent to glean, in some limited way, its durability as well. The Reliability Index Averages for each line, each manufacturer, the Reliable Two (Toyota and Honda), the Top Four (Toyota, Honda, Subaru and Nissan), the Middle Six (Mazda, Suzuki, Volvo, Mercedes Benz, BMW and Mitsubishi), the Big Three (General Motors, Ford and Chrysler) and the Bottom Four (Volkswagen, General Motors, Ford and Chrysler) were then computed, by Microsoft’s Excel software, for each pair of manufacturing years. These computations used those models for which there was sufficient data to ascribe a Reliability Index Value, except that the old foreign made and/or foreign engineered Geo line, and progeny, are omitted from General Motors’ averages and that the Mitsubishi made Colt and Stealth are omitted from Chrysler’s averages. Thus, in particular, the averages for the Reliable Two, the Top Four, the Middle Five, the Big Three and the Bottom Four are not averages of line or manufacturer averages, but are averages of the Reliability Index Values of the included models. Also note that all averages are unweighted by sales. One caveat to what follows: Counts and computations were many, so no doubt some errors exist, but hopefully no Reliability Index Value is off by more than .05. Should anyone find an error, please kindly advise. Your kind help in making for a more accurate web site will be sincerely appreciated by all.

The Reliability Score The definition of Reliability Score begins with two observations. First, the symbols in the right-most column of a Consumer Reports auto model reliability table depicts reliability performance of categories (called “trouble spots,” by Consumer Reports) of components of motor vehicles in the approximate age range of 0 to 1 year – the exact age range depending upon when the corresponding model year began and ended and when Consumer Reports stopped accepting returned questionnaires for its compilations. Second, similarly, the second column from the right, the third column, the fourth column, the fifth column, the sixth column, the seventh column, and the eighth column depict reliability performance of the same categories of components in the approximate age ranges of 1 to 2 years, 2 to 3 years, 3 to 4 years, 4 to 5 years, 5 to 6 years, 6 to 7 years, and 7 to 8 years, respectively. Next, some particular Reliability Scores are defined, from which may be seen a general definition. The Reliability Score RS(The Lexus LS400,1991,7-8) was obtained as follows: 1.      The leftmost column of the Lexus LS400 reliability table in the April 1999 issue of Consumer Reports was selected, as it is this column that rates problem frequencies by “trouble spots” in the 1991 Lexus LS400s that were in the age range of 7-8 years, approximately. 2.   The trouble spots of “Manual Transmission,” “Clutch,” “Brakes” and “Exhaust” were eliminated from this left-most column for reasons discussed in step 1 of the foregoing definitions of Reliability Index Value and Average. 3.   If all of the remaining 12 trouble spots had a Consumer Reports rating, the following was done. First, -1 was ascribed to each of those trouble spots that earned the lowest rating (had the highest frequency of reported auto problems). Second, - ½ was ascribed to each of those trouble spots that earned the second lowest rating (had the second highest frequency of reported auto problems). Third, 0 was ascribed to each of those trouble spots that earned the middle rating (had the third highest frequency of reported auto problems). Fourth, + ½ was ascribed to each of those trouble spots that earned the second highest rating (had the second lowest frequency of reported car problems). Fifth, +1 was ascribed to each of those trouble spots that earned the highest rating (had the lowest frequency of reported auto problems). Next, the numbers that were obtained in 3 were added together and divided by 12. This value is the Lexus LS400’s Reliability Score: RS(The Lexus LS400,1991,7-8).  Rather at once, it is seen that this value falls in the closed interval [-1,+1], as is the case for all Reliability Index Values.

 

The Reliability Score continued The Reliability Score RS(The Lexus LS400,1991,6-7) was computed in an identical manner, save that the second from left column of the Lexus LS400 reliability table in the April 1998 issue of Consumer Reports was chosen, as it is this column that rates problem frequencies by trouble spots of the 1991 Lexus LS400s that were between the ages of 6-7 years, approximately. Next, the Reliability Score RS(The Lexus LS400,1991,6-8) was computed thusly: RS(The Lexus LS400,1991,6-8)=(RS(The Lexus LS400,1991,6-7)+RS(The Lexus LS400,1991,7-8))/2. It follows immediately that RS(The Lexus LS400,1991,6-8)Î[-1,+1]. Similarly, the Reliability Scores of all models in model years 1988 to 1997 were computed when data permitted, with certain qualifications and exceptions. Firstly, the 1996 Buying Guide and the 1997 Buying Guide were used in lieu of the corresponding April issues of Consumer Reports. Secondly, commencing with the April 2000 issue, the trouble spots of “Manual Transmission” and “Clutch” were eliminated by Consumer Reports, so it was unnecessary for site manager to do so. Thirdly, the RSs for the Geo line, and progeny, were not computed, those for the Colts and the Stealth were not computed, and those for the Isuzu Oasis were not computed, as these models are rather rebadged products of other auto companies whose products are of greater or much greater reliability. However, the Honda Passport and the Nissan Quest were included, as site manager thought it important to alert prospective purchasers of these atypical products offered by Honda and Nissan. The Reliability Scores appearing in the Motor Vehicle Reliability Score table are generally those from two years of surveys. Those scores based only on one year of data have a background softened by white diagonal lines. Line, auto manufacturer, and auto manufacturer group Reliability Scores appearing in the Motor Vehicle Reliability Score table are means of model RSs. In particular, an auto manufacturer RS is a mean of model RSs, not a mean of line RSs. Note that all line, auto manufacturer, and auto manufacturer group RSs are means unweighted by sales. Finally, the Motor Vehicle Reliability Score table is provided so that others may check my work. There number of operations involved in generating the Reliability Score, the Reliability Percentrank, and the Reliability Grade tables is immense, and although all operations have been double checked (or checked more frequently, especially if an error was found), the number of errors that have survived such a double check are unknown. All corrections are sincerely appreciated.

 

The Reliability Percentrank The Reliability Percentrank of a model, for a given model year and vehicular age range, is a rank of that model’s Reliability Score relative to all other Reliability Scores for the same model year and vehicular age range. A model’s Reliability Percentrank (RP) is rounded to two places to the right of the decimal; consequently, all Reliability Percentranks fall between 0.00 and 1.00. If a model has the lowest Reliability Score for a given model year and vehicular age range, and is not tied for that lowest score, its RP is 0.00 for the applicable model year and vehicular age range. If a model has the highest Reliability Score for a given model year and vehicular age range, and is not tied for that highest score, its RP is 1.00 for the applicable model year and vehicular age range. Microsoft’s Excel 2002 Percentrank function was used to compute a model’s RP, and consequently that program’s code defines it precisely. However, roughly, if a model’s RP is between .90 and 1.00, its Reliability Score is among the top ten percent of the Reliability Scores for the applicable model year and vehicular age range. Similarly roughly, if a model’s RP is between .80 and .90, its Reliability Score is among the top twenty percent of the Reliability Scores for the applicable model year and vehicular age range, but not among the top ten percent. Also similarly roughly, if a model’s RP is between 0.00 and .10, its Reliability Score is among the bottom ten percent of the Reliability Scores for the applicable model year and vehicular age range. One exception to these generalizations, and the pattern that they offer, occurs when several or many Reliability Scores, and consequently Reliability Percentranks, are clustered at a particular value. This clustering occurs often at the low end. While Percentranks computed using Microsoft’s Excel 2002 Percentrank function have not been studied by site manager, they appear to have at least one anomalous property. To describe this property, two definitions are introduced. Firstly, a model’s Reliability Percentile is defined as the percentage, expressed as a fraction rounded to two places to the right of the decimal, of all models whose Reliability Score is equal to or less than that of the particular model. Secondly, a model’s Reliability Percentike is defined as the percentage, expressed as a fraction rounded to two places to the right of the decimal, of all models whose Reliability Score is less than that of the particular model. Now, it appears that Percentrank Î [Percentike,Percentile]; however, the Percentrank function’s method for selecting a particular value in this range was not evident to site manager by casual observation. Also, note that if many Reliability Scores are clustered at one value, the corresponding Reliability Percentile and Reliability Percentike will differ by quite a bit.

 

The Reliability Percentrank continued The Reliability Percentrank RP(model,model year range) is a mean over the model years falling within the model year range. These RPs appear in columns on the far right of the Motor Vehicle Reliability Percentrank table. Line, auto manufacturer, and auto manufacturer group Reliability Percentranks appearing in the Motor Vehicle Reliability Percentrank table are means of model RPs that appear within the same column. In particular, an auto manufacturer RP is a mean of model RPs, not a mean of line RPs. Note that all line, auto manufacturer, and auto manufacturer group RPs are means unweighted by sales.

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