Use of Present participle and past tense in a sentence..

[mektok]

Hi, I'm not an English native speaker. I hope someone will help me on these.
Which one is correct? What is/are the reason (s)?
1. However, a few studies have reported that Cryptosporidium oocysts are not found in stools of healthy children.
2.However, a few studies have reported that Cryptosporidium oocysts were not found in stools of healthy children.
3.However, a few studies ( no have) reported that Cryptosporidium oocysts are not found in stools of healthy children.
4. For the last few decades, as many as 60% of childhood infections were reported to be asymptomatic.
5.For the last few decades, as many as 60% of childhood infections have been reported to be asymptomatic.
6.For the last a few decades, as many as 60% of childhood infections were reported to be asymptomatic.

When do we use "someone" and "somebody". Are these the same?

Thank you,

ciao

[5jj]

1. However, a few studies have reported that Cryptosporidium oocysts are not found in stools of healthy children.
This is the tense we'd expect. It is reporting the situation.

2.However, a few studies have reported that Cryptosporidium oocysts were not found in stools of healthy children.
Acceptable. It is reporting what was found.

3.However, a few studies ( no have) reported that Cryptosporidium oocysts are not found in stools of healthy children.
Acceptable; 'were' is more likely.

4. For the last few decades, as many as 60% of childhood infections were reported to be asymptomatic.
Unlikely. 'For the last few decades' brings the time up to the present. The present perfect is appropriate

5.For the last few decades, as many as 60% of childhood infections have been reported to be asymptomatic.
Fine.

6.For the last a few decades, as many as 60% of childhood infections were reported to be asymptomatic.
.

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