The United Nations has declared that people with disabilities should be enabled to live as independently as possible, since independence is correlated with a better quality of life. Consequently, services need to have common and validated measurement tools for the evaluation of the different levels of personal support needs in order to promote independent living skills. We aimed to create and validate the Adult Independence Living Measurement Scale (AILMS) to estimate personal skills considered tantamount for independent living in adult persons with intellectual and developmental disabilities.

AILMS is a short informant‐rated assessment tool consisting of 19 items (goals) regarding the most important skills related to independent living. AILMS total score is directly proportional to the degree of independence, with scores ranging from 19 to 76. Our validation is a multicentre study attended by 243 subjects, 110 female and 123 males, with a median age of 37 years and with an interquartile range (IQR) of 18 (25th percentile [Q1] 29 years to 75th percentile [Q3] 47 years). All subjects had a diagnosis of intellectual disability associated with various neurodevelopmental disorders or syndromic conditions.

The AILMS shows a wide range of scores with a minimum score of 21 and a maximum of 72. We found no floor or ceiling effects for the total score on the AILMS. Cronbach's α coefficient (= 0.95), based on the 19 AILMS items, indicated high internal consistency. The tool demonstrates a very good agreement even when comparing the results submitted by two different interviewers. It also shows an excellent temporal stability of 1 week, with intraclass correlation coefficients both of 0.97. AILMS total scores do not differ by sex or age, while statistically significant differences are observed between people with different levels of severity of ID. Convergent validity of AILMS was analysed by correlating its total scores with the Italian validated versions of the Support Intensity Scale (SIS‐I) and the Alzheimer's Functional Assessment Tool (AFAST‐I) scores. Strong inverse Spearman correlations coefficients (r_s) were found both for the Support Need Index of the SIS‐I (r_s = −0.66; P < 0.001) and AFAST‐I (r_s = −0.73; P < 0.001).

Scores of support needs in exceptional health disorders of the SIS‐I appear unrelated to AILMS total scores (r_s = −0.01; P = 0.05), confirming the divergent validity of the new scale. Exploratory factor analysis reveals three underlying factors within the AILMS, with factor 1 explaining 51.2% of the total variance (Cronbach's α = 0.92) composed of predominantly nine advanced daily activities.

The AILMS has good psychometric properties and user friendliness and may therefore be a valuable addition to the current informant‐rated tools for screening and assessment of independent living skills of individuals with intellectual and developmental disabilities.