It should also be noted that the results reported in this paper include a significantly expanded corpus of tweets (nearly an order of magnitude greater) as we continued collecting data following the publication of our original work. 

The remainder of this paper is structured as follows. In the next section we describe some of the key aspects of the autism spectrum disorder which motivated our focus on this particular condition. In Section 3 we review relevant previous work on the use of Twitter for data mining and highlight the key techniques and methods in the field. Section 4 describes the main contribution of our work – we start by describing the manner in which the data used for the present study was collected and follow up with a description, results, and discussion of a sequence of increasingly complex experiments. A summary of the work and its findings is made in Section 5. 

## 2 Motivation and relevant background 

Autism spectrum disorder is a life-long neurodevelopmental disorder with poorly under- stood causes on the one hand, and a wide range of potential treatments supported by little evidence on the other. The disorder is characterized by severe impairments in social interac- tion, communication, and in some cases cognitive abilities, and typically begins in infancy or at the very latest by the age of three. ASD is recognized as comprising an aetiologically and clinically heterogeneous group of conditions whose diagnosis remains to be based solely on the complex behavioural phenotype [2]. According to the definition in the latest version (5th edition) of the Diagnostic and Statistical Manual of Mental Disorders, the autism spec- trum disorder includes disorders which were previously diagnosed with more specificity as autism, Asperger syndrome, Rett syndrome, childhood disintegrative disorder, and ‘per- vasive developmental disorder not otherwise specified’ [3]. Current evidence suggests that approximately 0.5-0.6% of the population is afflicted by ASD though the actual diagnosis rate is on the increase due to the broadening diagnostic criteria [4]. The condition is usually detected in early childhood when an abnormal lack of social reciprocity is observed. 

Considering the social and economic burden of ASD it is unsurprising that it has been attracting an increasing amount of research attention; numerous longitudinal, epidemiolog- 

