The success rate of speech therapy would depend on a few factors. One factor would be the speech-language disorder being treated (and its severity). There are many different speech-language disorders that may require speech-language therapy including the following: speech sound/articulation disorder, childhood apraxia of speech, orofacial myofunctional disorders, stuttering/fluency disorder, mixed receptive/expressive language delay or disorder, pragmatic (social skills) language disorder, resonance/voice disorders, dysarthria, aphasia, and dysphagia.
If a child is receiving speech therapy for a mild articulation disorder and needs help correcting just a couple sounds, it is likely therapy will be successful. Stuttering or other fluency disorder may be more difficult to completely correct, however, if the client is able to carryover strategies learned, therapy is considered successful.
The age of clients/patients also is a factor for successful therapy. A young child who has received therapy for a couple years may respond better to therapy than an elderly client receiving therapy for a swallowing disorder.
Success rate of speech therapy would also depend on guardians and/or parents completing the carryover work in the home environment. If a 2 year old if receiving therapy due to language delay where he/she is not yet using many words to communicate, it is very important for the parent/guardian to work on skills recommended by the speech-language pathologist in order for therapy to be successful.
One of the biggest factors for how successful speech therapy is may be how early treatment begins. Early intervention plays a big role in the success rate for speech therapy. If a 2 year old is not yet using words to communicate, it is best they begin treatment then and not wait for the child to maybe start talking at age 3. According to Heathline, speech therapy for young children has been shown to be most successful when started early and practiced at home with the involvement of a parent or caregiver.
In one study, 70 percent of preschool kids with language issues who went through speech therapy showed improvement in language skills.
https://www.understood.org/en/learning-thinking-differences/treatments-approaches/therapies/what-you-need-to-know-about-speech-therapy https://www.healthline.com/health/speech-therapy